By X. Dan. Southern University, Shreveport-Bossier City. 2019.
Flea-Borne Endemic Typhus Flea borne or murine typhus is caused by Rickettsia typhi (mooseri) which is ingested by the flea with its blood-meal order neurontin 100mg line. Within the gut the rickettsiae multiply neurontin 600mg online, but unlike plague bacilli they do not cause any blockage of the proventriculus or stomach order 100mg neurontin with mastercard. Infection is caused by infected faeces being rubbed in to abrasions or coming in to contact with delicate mucous membranes order generic neurontin, and also by the release of 158 rickettsiae from crashed fleas. Murine typhus is essentially a disease of rodent, particularly rats and especially Rattus rattus and R. People become infected mainly through Xenopsylla cheopis, but occasionally Nosopsyllus fasciatus, Ctenocephalides canis; C. Leptopsylla segnis does not attack humans, but it is possible that murine typhus is sometimes spread to them by an aerosol of infective faeces of this species. The rickettsia of murine typhus can pass across in the fleas ovaries, in to the eggs and then to the larvae, a form of trans-ovarial transmission. Cestodes Dipylidium caninum is one of the more common tapeworms of dogs and cats and occasionally occurs in children, and Hymenolepis diminuta infects rats and mice and occasionally people. Eggs of these parasites are passed out with excreta of rats and domestic pets and may be swallowed by larval fleas feeding on excreta. Larval worms hatching from the ingested eggs penetrate the gut wall of the larval flea and pass across in to the body cavity (coelom). They remain trapped within this space and pass on to the 159 pupa and finally to the adult flea where they encapsulate and become cysticercoids (infective larvae). Animals can become infected by licking their coats during grooming and thus swallowing the infected adult fleas. Similarly, young children fondling and kissing dogs and cats can become infected with D. Hymenolepis nana is known to be capable of developing in insects, and it is possible that fleas and rodents some times serve intermediate as hosts and reservoirs of infection, respectively 5. Less Important Diseases In addition to the above diseases and parasites spread by fleas, these insects may play some small part in the transmission of Francisella tularensis, Rickettsia conori, Coxiella burneti and some other rather minor infections. However, it should be stressed that their role as vectors of these pathogens is at the most minimal. It has occasionally been reported in persons in India returning from overseas, mainly Africa, but the flea is not indigenous to India or elsewhere in Asia. Tunga penetrans does not transmit any disease to people but is a nuisance because females burrow in to the skin. They have neither general nor pronotal combs and are easily separated from other fleas of medical importance by their very compressed first three (thoracic) segments and the paucity of and bristles on the body. Life Cycle and Medical Importance Eggs are dropped on to the floor of houses or on the ground outside. They hatch within about 3-4 days and the larvae inhabit dirty and dusty floors or dry sandy soils especially in areas frequented by hosts of the adult fleas. Under favorable conditions larval development is completed within about 10-14 days. The pupal period lasts about 5-14 days, and complete life cycle can be as about 18 days. Newly emerged adults are very agile and jump and crawl about on the ground until they locate a suitable host which is usually a person or pig. Both sexes feed on blood but whereas the male soon leaves the host after taking a blood-meal. The female after being fertilized 161 burrows in to the skin where it is soft as between the toes or under toe- nails. In people habitually sitting on the ground such as beggars or infants the buttocks may often be infected and particularly heavy infections have been recorded from leprosy patients. In heavily infected individuals the arms especially the elbows may also be attacked and occasionally the females burrow in to the soft skin around the genital region. Burrowing in to the skin appears to be accomplished by the fleas sharp and well-developed mouthparts. The result is that the entire flea with exception of the abdomen bearing the anus genital opening and large respiratory spiracles becomes completely buried in the hosts skin. The area surrounding the embedded portion results in ulcerations and accumulation of pus. While the blood-meal is being digested the abdomen distends to an enormous size (about 1000 times a small pea. Towards the end of this period of abdominal enlargement the ovaries are composed of thousands of minute eggs. Over the next 7-14 days about 150-200 eggs are passed out of the female genital opening most of which eventually fall to the ground and hatch after about 3-4 days. This frequently causes inflammation and may in addition result in secondary infections which if ignored be able to lead to loss of the toes tetanus or even gangrene. Because they are feeble jumpers wearing shoes is a simple but in some communities relatively costly 162 method of the likelihood of flea infection. Females embedded in the skin should be removed with fine needles under aseptic conditions and wounds formed by their extraction is sterilized and dressed. They are removed with in the first few days, before they are becoming established as they are difficult to extract when they have greatly distended abdomens containing numerous eggs without rupturing them and this increases the risk of infections. Pigs in addition to humans are often commonly invaded by Tunga and they may provide a local reservoir of infection. Flea collars remain effective for 1-2 months, but they may cause skin problems in the animal. Alternatively insecticides like fenthion can be formulated as a Spot-on solution that is applied to the pets skin in just one small area. The insecticide is absorbed through the skin and passes to the animals blood so that blood-feeding fleas ingest the insecticide. However an important consideration is that most fleas are found away 163 from the host not on it. For example it has been said that a typical colony of cat fleas consists of only about 25 adult fleas on the cat than on the floor and bedding. There may be 500 adult fleas 500 cocoons and as many as 3000 larvae and 1000 eggs. Clearly control measures should not be restricted to the cat but applied to the total environment. Flea cocoons are not very susceptible to insecticides consequently insecticidal treatments should be repeated about every 2 weeks for about 6 months. Beds, kennels or other places where pets sleep or spend much of their time should be treated either with insecticidal powders or lightly sprayed with solutions containing 0. In such cases organophosphate or carbamate insecticides such as 2% diazinon, 2% fenthion (Baytex), 5% Malathion, 2% fenitrothion (sumithion) 5% iodofenphos (jodfenphos) or 3-5% carbaryl (sevin) can be used. Insecticidal fogs or aerosols containing 2% malathion or 2% Ronnel have sometimes been used to fumigate houses harbouring fleas. For the control of fleas in urban outbreaks of plague or murine typhus 164 extensive and well-organized insecticidal operations may be necessary. At the same time as insecticides are applied rodenticides such as the anti-coagulants, for example warfarin and fumarin can be administered to kill the rodent population. However if fast- acting one- dose rodenticides such as zinc phosphide, sodium flouroacetate or strychnine or the more modern fast-acting anticoagulants like bromadiolone and chlorophacinone are used then it is essential to apply these several days after insecticidal applications. Otherwise the rodents will be killed but not their fleas which will then bite other mammals including people and this may result in increasing disease transmission. Insect growth regulators More recently insect growth regulators such as methoprene have been incorporated in to sprays often combined with a pyrethriod such as permethrin for treating household infestations such sprays may remain effective for 10 week. Insect growth regulators are virtually non-toxic to pets and other mammals and some have been formulated as tablets or syrup that is given to cats or dogs orally. Methoprene kills fleas by interfering with their cycle of development for example by preventing larvae reaching adulthood or eggs from hatching. Other insect growth regulators are chitin- inhibitors that prevent the insects exoskeleton formation properly.
Polynesiensis occur in man made and natural containers cheap neurontin uk, especially split coconut shells buy generic neurontin canada, whereas larvae of Ae buy generic neurontin 300mg on line. Both of these species are important vectors of diurnal subbperioldic bancroftian filariasis cheap 800 mg neurontin with mastercard. Aedes togoi, a vector of nocturnal periodic bancroftian and burgian filariasis, breeds principally in rock-pools containing fresh or brackish water. The life cycle of Aeds Mosquitoes from eggs to adults can be rapid, taking as little as about 7 days, but it more usually takes 10-12 days; in temperate species the life cycle may last several weeks to many months, and some species over winter as eggs or larvae. Most biting occurs out of doors and adults usually rest out of doors before and after feeding. Eggs Eggs are usually black, more or less ovoid in shape and are always laid singly. Eggs are laid on damp substrates just beyond the water line, such as on damp mud and leaf litter of pools, on the damp walls of clay pots, rock-pools and tree-holes. When flooded, some eggs may hatch within a few minutes, others of the same batch may require prolonged immersion in water, thus hatching may be spread over several days or weeks. Even if environmental conditions are favorable, eggs may be in a state of diapauses and will not hatch until this resting period is terminated. Various stimuli including reduction in the oxygen content of water, changes in day length, and temperature may be required to break diapauses in Aedes eggs. Many Aedes species breed in small container- habitats (tree-holes, plant axils, etc) which are susceptible to drying out, thus the ability of eggs to withstand desiccation is clearly advantageous. Desiccation and the ability of Aedes eggs to hatch in installments can create problems with controlling the immature stages Larvae Aedes species usually have a short barrel-shaped siphon, and there is only one pair of sub ventral tufts which never arise from less than one-quarter of the distance from the base of the siphon. Additional characters are at least three pairs of setae in the ventral brush, the antennae are not greatly flattened and there are no enormous setae on the thorax. These characters should separate Aedes larvae from most of the culicine genera, but not unfortunately from larvae of South American Haemagogus. In central and South America, Aedes larvae can usually be distinguished from those of Haemagogus, by possessing either larger or more strongly speculate antennae; also the comb is not on a sclerotized plate as in some Haemagogus. Aedes aegypti, often called the yellow fever mosquito, is readily recognized by the lyre shaped silver markings on the lateral edges of the scutum. Scales on the wing veins of Aedes mosquitoes are narrow, and are usually more or less all black, except may be at the base of the wing. In Aedes the abdomen is often covered with black and white scales forming distinctive patterns, and in the female it is pointed at the tip. Yellow fever Yellow fever is a zonoosis, essentially a disease of forest monkeys, which occasionally transmitted to humans. The yellow fever virus 54 mainly occurs in population of monkeys in dense forests and the disease is transmitted from monkey to monkey by forest dwelling mosquitoes called Aedes africanus in Africa, heamagogus and sabeths in south and central America Yellow fever. There are two epidemiological types of the disease, urban and jungle yellow fever. The same virus causes both types, but the mosquito vectors and vertebrate hosts are quite different. Transmission In Africa the yellow fever virus occurs in certain cercopithecid monkeys inhabiting the forest and is transmitted amongst them mainly by Aedes africanus. This is a forest-dewlling mosquito that breeds in tree-holes and bites mainly in the forest canopy soon after sunset just in the right place at the right time to bite monkeys going to sleep in the tree-tops. Some species of monkeys involved in the forest cycle, such as the red-tailed guenon, descend from the trees to steal bananas from farms at the edge of 55 the forest in this habitat, the monkeys get bitten by different mosquito including Aedes bormeliae (formerly called Ae. This species bites during the day at the edges of forest and breeds in leaf axils of bananas, plantains and other plants such as coco- yams (Colocasia) and pineapples. If the monkeys have viraemia, that is yellow fever virus circulating in their peripheral blood, Ae. Bromeliae becomes infected, and if the mosquito lives long enough it can transmit yellow fever to other monkeys or more importantly to people. This transmission cycle, occurring in clearings at the edge of the forest involving monkeys, Ae. When people return to their villages they get bitten by different mosquitoes, including Ae. Aegypti, a domestic species breeding mainly in man made containers such as water-storage pots, abandoned tin cans and vehicle tyres. There is increasing evidence in West Africa that in rural areas other Aedes species spread the virus from monkeys to people. In some areas for example, yellow fever may be circulating among the monkey population yet rarely gets transmitted to humans because local vector mosquitoes are predominantly zoophagic. Other primates in Africa such as bush- 56 babies (Galago species) may also be reservoirs of yellow fever. There is some evidence from West Africa that yellow fever virus may be trans-ovarially transmitted in Aedes species, as males have been found infected with the virus. Thus, after 4 or 5 days the virus appears in the peripheral blood, that is viraemia is produced, and this occurs irrespectively of whether monkeys or humans are showing overt symptoms of the disease. Viraemia lasts only 2-3 days, after which the virus disappears from the peripheral blood never to return and the individual is immune. Monkeys and people are therefore infective to mosquitoes for only about 2-3 days in their entire lives. A relatively high titre of yellow fever (and also any other arbovirus) is needed before it can pass across the gut cells of the mosquito into the haemolymph, from where it invades many tissues and organs, including the salivary glands, where virus multiplication occurs. A mosquito must therefore live a sufficiently long time before it becomes infective and capable of transmitting an arbovirus-or malaria or filariasis. It is characterized by a sudden high fever, severe headache, backache and pain in the joints. There are four strains or types 58 and at least one or all four are found throughout much of the world. Recovering victims are generally immune to future infections, but only from the strain they were infected with. Therefore, a person can potentially experience all four different strains of dengue. Important Species of Aedes Aedes aegypti is a small dark species easily identified by the lyre- shaped, silvery-white lines on the thorax and the white bands on the tarsal segments. It is a vector of urban yellow fever and dengue, and it is a pest when present in large numbers. Aedes aegypti is essentially a tropical species, probably introduced into the Western World from Africa. This is a thoroughly domesticated mosquito and breeds almost exclusively in artificial containers in and around human habitations. The females lay their eggs singly on the water just at the margin, or on the sides of the container above the water line. They prefer human blood to that of other animals and readily enter homes to find suitable hosts. It attacks quietly, preferring to bite around the ankles, under shirt sleeves, or on the back of the neck. Adults can have a golden-brown color on top of the thorax and a longitudinal stripe of white or yellowish-white scales on the abdomen. Females lay their eggs singly on the mud of salt marshes where they remain until 59 flooded by high tides or rains. They usually use pot holes and depressions of various sizes, but they may also lay eggs over rather extensive level areas. After having been dry for a week or two, they will hatch in a few minutes when water covers them. Aedes sollicitans adults are strong fliers and often migrate in large swarms from marshes to cities and towns many miles away. Migratory flights begin just before dark and may include tremendous numbers of mosquitoes. They rest among the grasses during the day but will readily attack anyone who disturbs them.
Localized accumulation of fluid in the peritoneum is cavity order 800 mg neurontin amex, particularly in the pelvis and under the diaphragm order neurontin with a mastercard. Also cheap 300mg neurontin otc, the nomenclature with regard to cavity effective neurontin 300 mg, more collections may still form postoperatively. This fluid, and is extremely porous to bacteria; thus septicaemia and that which is lost into the abdominal cavity, depletes quickly ensues. So it is not surprising that the mortality the circulating blood volume so the urine output falls, and from generalized peritonitis is at best 10%, even in good the pulse rate rises. Traumatic or surgical diagnosis, and examination should merely confirm or intervention obviously allows a route for bacteria to refute it. After the operation, if the diagnosis is a surprise, contamination of the abdominal cavity, when you perform think back to distinguish which features of the history were a laparotomy. However, dont wait till you are certain of the bowel carefully, pack away potentially infected areas, diagnosis before you operate on a patient who needs control bleeding meticulously, and use appropriate surgery acutely! Not to examine the patient carefully and Pain may also be referred from the diseased area to the systematically, admit him and monitor him carefully and other parts of the body that are derived from the same to look at him again if you are not clear of the diagnosis, spinal segment. Not to make and record a diagnosis and a differential under the diaphragm may present with pain in the diagnosis, especially if you are handing over to a shoulder. To forget that many medical conditions, especially There can be pain of more than one kind. For example, pneumonia (by causing diaphragmatic pain) can mimic an when the lumen of the appendix is obstructed, there is acute abdomen. If the appendix becomes probability of a particular diagnosis gangrenous, there is ischaemic pain of type (2). Not to make adequate allowance for the late case whose history is obscured, whose mind is clouded, and whose signs are altered. Constitutional disturbances such as anorexia, nausea and vomiting, the inability to pass flatus in the presence of either constipation or diarrhoea and frequency of micturition are common. In general, the patient with peritonitis is weak, thirsty, anorexic and nauseated. B, (6) kidney and pancreatic pain its correct interpretation will lead you towards the cause. C, (8) diaphragmatic pain is frequently is highly significant and demands you find out the cause! D, when you examine the abdomen for Look out for the pattern of pain: tenderness, alwayslook at the patients face: wincing is a very reliable sign of peritoneal irritation. Suddenly: Rupture of duodenal ulcer or bowel (3) A colicky pain is due to obstruction of either bowel, Torsion of testicle, bowel or ovarian cyst biliary tree or urinary tract: it comes in waves and spasms Mesenteric vessel occlusion and often makes the patient move about restlessly. Lying absolutely still Peritonitis Have you noticed any change in your bowel habit Walking bent forwards Appendicitis, pancreatitis Usually regular, constipation for Large bowel Lying with knees flexed Appendicitis, psoas abscess several days obstruction (Inflammation in contact Hypogastric pain & diarrhoea Pelvic abscess with the psoas muscle) with mucus, then hypogastric tenderness & constipation What makes it worse? Was it before or after the give you some clues, especially with intestinal obstruction normal time? Vigorous The temperature is raised; if it is not, the inflammation is massage of the abdomen may cause bowel injury! Ask the patient to point to where the (which you may note simply by a difficulty in speaking), pain started, to where it is now, and to where it is worst. Tachycardia is common as is sweating whereby the Look for distension due to gas, or fluid: test for shifting extremities may be warm or cold. Initially in peritonitis the abdomen is tympanitic, present and infants as well as the aged may have later it will fill with ascitic fluid. Peritonitis The general condition may be surprisingly normal anywhere may splint all or part of the abdomen, and stops especially early on in the disease. If he is restless, Visible peristalsis means there is obstruction not paralytic suspect cerebral hypoxia, due to hypovolaemia. Look for a dry tongue and lack of skin turgor Expose the whole abdomen including genitalia & look at (dehydration), pale conjunctivae (anaemia), mouth signs of the groins: there may be an obstructed irreducible hernia. Nose tip & earlobe and hands Hypovolaemia The great Hamilton Bailey (one of Britains foremost cold Peripheral circulatory failure surgical teachers, 2. If extending Deathly pale with gasping Severe bleeding the hips causes abdominal pain, this is a reliable sign of respiration (e. Tap the abdomen with your Gaze dull and face ashen Severe toxaemia fingers: if this causes pain, you dont need to hurt the Eyes sunken, tongue and lips Dehydration, patient further by pressing harder! Lay your hand flat on the abdomen, and keep your fingers dry, skin elasticity reduced Intestinal obstruction. Your hand must (The Hippocratic facies is a combination of all of these; be warm, gentle, patient, and sensitive. Tachycardia is usual in peritonitis, and early in Then, if necessary use deep palpation. The pulse of typhoid fever is Wincing (10-1D) on pressure of the abdomen is a very no longer slow after the ileum has perforated. If peritonitis is advanced, there The patients attitude in bed is characteristic: lying still is no need to test for rebound tenderness; it is cruel and and only changing position in bed with pain and difficulty, unhelpful. But if peritonitis is localized, straight one minute and doubled up the next, he has colic rebound tenderness is a good indication as to which parts and not peritonitis. Get him to lie on the opposite side to where the pain is, Find where the area of greatest tenderness is. It will be and extend the thigh on the affected side to its fullest easier to find if there is no generalized guarding, and is a extent. If this is painful, there is some inflammatory lesion useful clue to the organ involved, e. If rotating the flexed thigh so as to peritonitis, especially if ascites is present. The peritonitis is advanced and ascitic fluid dilutes the The thoracic percussion test. Percuss gently with your fist peritoneal irritation: so as he gets more ill, the tenderness over the lower chest wall. Percuss for liver dullness in the right nipple line from the (5);He has been given narcotic analgesia, especially 5th rib to below the costal margin. If liver dullness is postoperatively, or is paraplegic with a sensory level at the absent, there is probably free gas in the abdominal cavity. Look for superficial induration and tenderness in the abdominal wall (pyomyositis, necrotizing fasciitis). A tender mass adjacent to the midline (haematoma from rupture of the epigastric artery) will not disappear when the patient tenses the abdominal wall (Fothergills sign). Ask the patient to flex the hip against the resistance Listen to the abdomen for decreased or absent bowel of your hand. If he feels pain, there is inflammation in relation to the sounds: you may need to wait for 3mins! Flex the hip to 90 and gently rotate it tinkling bowel sounds, coinciding with worsening of internally and externally. If this causes pain, there is inflammation in abdominal pain, are a sign of obstruction not peritonitis. On the right a sharp pain indicates Diminished bowel sounds means nothing specific! It may only feel like slightly under the left hemi-diaphragm (and also to look at the thickened cord and testicle, with reddening and oedema of condition of the lungs). Lay the patient in the Signs on abdominal films (erect & supine) are subtle: lateral position. Pass a well-lubricated finger as far up the (1);Air in the small bowel: this is always abnormal except anal canal as it will go. Feel upwards for a stricture, (2);Displacement of the colon: a ruptured spleen may the apex of an intussusception, or the bulging of an abscess displace the shadow of the splenic flexure downwards and against the rectal wall. Feel bimanually for a (3);Obliteration of the psoas shadow: this can be caused by pelvic tumour or swelling, or for any fullness in the pouch pyomyositis of the psoas, a psoas abscess from a of Douglas. Look if there is blood or mucus on your glove tuberculous spine, a retroperitoneal abscess or by bleeding afterwards. Examine the (5);Urinary calculi: look for these along the lines joining spine (spinal tuberculosis or a tumour can cause root pain the tips of the transverse processes of the vertebrae to the felt in the abdomen). Look at the testes to (6);A faecolith in the area of the appendix, when there are exclude torsion.
Remember cheap 300mg neurontin free shipping, any container that will hold water for 5 to 7 days is a potential breeding site for mosquito larvae cheap neurontin 100mg fast delivery. If it is not possible to drain areas buy 300mg neurontin amex, then treating them with larvicides if they are supporting larvae is an alternative generic neurontin 300mg on line. Many mosquito larvicides on the market are very host specific and only disrupt the larval stage of mosquitoes and do not harm non target species. They can also avoid being outside at dawn, early evening, and dusk when the majority of biting 81 female mosquitoes are active. This type of control is not usually effective in Oklahoma because it is very rare for conditions to be conducive for fogging adult mosquitoes. Adult mosquitoes must come into contact with the pesticide, so timing of application is critical. Since different mosquito species are active during different periods throughout a 24hr day it is critical to fog at exactly the proper time to get effective control of the target species. Extensive behavioral knowledge of the species to be controlled must be utilized and the spray applied only when adults are active. Weather conditions must be considered, as windy conditions usually present in the spring in Oklahoma may cause pesticides to drift out of an area so that they never reach their intended target. Pesticide label rates and recommendations must be followed according to the manufacturers instructions for all pesticides applied. Mosquito control products available for consumer use are typically sold through local home and garden supply stores. Product availability may vary according to location and state regulation Review Questions 1. Write at least three distinguishing features of mosquito from other biting dipteras. Describe the medical importance, and control methods for anopheles, culex, and aedes 3. Justify the reasons why malaria is still the most prevalent disease in the tropics. There are more than 1300 known species of black flies (or "buffalo gnats," as they are sometimes called) in the family Simulidae. As their vernacular name indicates they are usually black in colour but many have contrasting patterns of white, silvery or yellowish hairs on their bodies and legs, and others may be predominantly or largely orange or bright yellow. Black flies have a pair of large compound eyes, which in females are 85 separated on the top of the head ( a condition known as dichoptic); in the males the eyes occupy almost all of the head and touch on top of it and in front, above the bases of the antennae ( a condition known as holoptic). The mouth parts are short and relatively inconspicuous but the five-segmented maxillary palps, which arise at their base, hang downwards and are easily seen. The arrangement and morphology of the mouth parts in similar to those of the biting midges (Ceratopogonidae) the mouthparts, being stout and broad, do not penetrate very deeply into the hosts tissues. Teeth on the labrum stretch the skin and the rasp-like action of the maxillae and mandibles cuts through it and rupture the fine blood capillaries. This method of feeding is ideally suited for picking up the microfilariae of Onchocerca volvulus which occur in human skin not blood. The relatively short legs are also covered with very fine and closely appressed hairs and may be uni- colorous or have contrasting bands of pale and dark colour. The wings are characteristically short and broad and (lack scales or prominent hairs. Only the veins near the anterior margin are well developed, the rest of the wing is membranous and has an indistinct venation. The abdomen is short and squat, and covered with inconspicuous closely appressed fine hairs. They are always laid in flowing water but the type of breeding place differs greatly according to species. Habitats can vary from small trickles of water, slow flowing streams, lake outlets and water flowing from dams to fast- flowing rivers and rapids. Some species prefer lowland streams and rivers whereas others are found in Mountain Rivers. Usually some 150-800 eggs are laid in sticky masses or strings on a level with, or just below, the water line on submerged objects. Femalses may crawl underneath the water and become completely submerged during ovipositing. There may be a few favoured oviposition sites in a stream or river, resulting in thousands of eggs from many females being found together. Eggs of species inhabiting temperate and cold northern areas may not hatch for many weeks and some species pass the wither as diapausing eggs. The body is slightly swollen beyond the head and in most, but not all, species distinctly swollen towards the end. The rectum has finger-like rectal organs which on larval preservation may be extruded and visible as a protuberance from the dorsal surface towards the end of the abdomen. Ventrally, just below the head, is a small pseudopod called the proleg which is armed with small circles of hooklets. Larvae do not swim but remain sedentary for long periods on submerged vegetation, rocks, stones and other debris. Attachment is achieved by the posterior hook-circlet (anal sucker of many previous authors) tightly gripping a small silken pad. This has been produced by the larvas very large salivary glands and is firmly glued to the substrate. This is achieved by alternatively attaching themselves to the substrate by the proleg and the posterior hook-circlet, thus they move in a looping manner. When larvae are disturbed they can deposit sticky saliva on a submerged object, release their hold and be swept downstream for some distance at the end of a silken thread. They can then either swallow the thread of saliva and regain their original position, or reattach themselves at sites further downstream larvae normally orientate themselves to lie parallel to the flow of water with their heads downstream. They are mainly 89 filter-feeders, ingesting, with the aid of large mouthbrushes, suspended particles of food. However, a few species have predacious larvae and others are occasionally cannibalistic. Larval development may be as short as 6-12 days depending on species and temperature, but in some species may be extended to several months, and in other species larvae overwinter. This cocoon is firmly stuck to submerged vegetation, rocks or other objects and its shape and structure vary greatly according to species. The pupa has a pair of, usually prominent, filamentous or broad thin- walled, respiratory gills. Their length, shape and the number of filaments or branches provide useful taxonomic characters for species identification. These gills, and the anterior part of the pupa, often project from the entrance of the cocoon. In both tropical and non-tropical countries the pupa period lasts only 2-6 days and is unusual in not appearing to be dependent on temperature. On emergence adults either rise rapidly to the water surface in a protective bubble of gas, which prevents them from being wetted, or they escape by crawling up partially submerged objects such as vegetation or rocks. A characteristic of many species is the more or less simultaneous mass emergence of thousands of adults. The empty pupa cases, with gill filaments still attached, remain 90 enclosed in their cocoons after the adults have emerged and retain their taxonomic value. Consequently, they provide useful information on the species of simuliids that have recently bred and successfully emerged from various habitats. Eggs, however, are never found on these animals; they are probably laid on submerged stones or vegetation. The nuclei of the larval salivary gland cells have large polytene chromosome which have banding patterns that are used to identify otherwise morphologically identical species within a species complex. For example, chromosomal studies have shown that there are about 40 cytologically different entities in the s. Biting occurs out of doors at almost any daylight hour, but teach species may have its preferred times of biting. Many species seem particularly active on cloudy, overcast days and in thundery weather.