26 ENTOMOLOGY FOR MEDICAL OFFICERS 



or veins; these are tubes of chitin containing air, and in 

 their developmental stages, blood. The functional hardened 

 wing is practically a plate of chitin. The form, density, 

 pose, clothing, and folding of the wings show great diversity 

 throughout the class, and furnish characters of systematic 

 importance. 



The true appendages of the thorax are the legs, of which, 

 in the adult insect, there are typically three pairs, one pair 

 on each segment. Each leg commonly consists of the 

 following successive parts : the coxa, which is articulated 

 to the thorax ; the trochanter, which is usually short ; the 

 femur and the tibia, both of which are usually long ; the 

 tarsus, which generally consists of several separate pieces, 

 often five, but sometimes fewer. The last tarsal segment 

 commonly ends in a pair of claws, but sometimes in a 

 single claw. The form and character of the legs show 

 great diversity in adaptation to different functions — running, 

 hopping, swimming, clinging, digging, seizing and holding 

 prey, collecting and carrying food, etc. There are insects 

 in which some of the legs are reduced in size and useless 

 for locomotion, or with some of the segments of the legs 

 absent. Rarely are legs altogether wanting. 



The abdomen contains the stomach and intestine, the 

 excretory tubules, and the reproductive organs. Stumpy 

 legs {pseudopods) are present on some of the abdominal 

 segments of certain larvae ; but in the adult insect this region 

 is destitute of true appendages, though the caudal fila- 

 ments {anal cerci) and parts of the ovipositors and stings 

 of certain insects are regarded as homologous with ap- 

 pendages. The abdomen of many Hymenoptera is peculiar 

 in often being constricted or even petiolated at its junction 

 with the thorax, and in being something less than an 

 abdomen, since the segment in front of the constriction 

 is united with — and appears to be part of— the thorax. 



The alimentary canal runs through the body and opens 

 posteriorly in the last visible abdominal segment ; its structure 

 varies according to the food it has to deal with. In most 

 insects that suck liquid food the pharynx, by the contraction 

 of muscles attached to its outer wall, acts as a suction pump, 

 and the oesophagus is provided with crops or food-reservoirs. 



