OESTRID LARVAE 317 



5. Palpi wanting .-'.-": : ' .= . " . . . Hypoderma. 



Palpi small, globular . , . . . . . . . Oedamagena. 



6.. Arista bare, short and stout ; wings without stump [at angle of 



fourth vein . . . . . . . Bogeria. 



Arista pectinate above , . . ' . . . . . . . . 7 



7. Tarsi broad and flattened, hirsute ; alulae large . . Cuterebra. 



Tarsi slender, not hirsute ; third joint of antennae more 

 elongate ; front prominent anteriorly ; alulae of moderate 



size Dermatobia. 



The body of the Oestrid larva is composed of twelve segments, the 

 first two of which cannot be differentiated from the cephalic ring ; no 



head can be recognized. The anterior stigmata ap- 



11 1 u c. j i- n u-j Structure of Oestrid 



pear as small knobs or fissures and are partially hid- |apya 



den in the folds of skin (Gastrophilus). The posterior 

 stigmata may be protrusile or retractile. In the mature Gastrophilus 

 larva and in that of Dermatobia they are protected by lip-like organs on 

 the last abdominal ring ; in all the other genera they can be simply 

 withdrawn into the preceding ring ; they consist of chitinous, cresentic or 

 reniform plates. 



All the young larvae have mouth parts ; in the later stages some have 

 oral hooks w r hile others have none. In Hypoderma the appearance 

 changes after the first moult and the oral hooks and mouth parts 

 disappear. The antennae are rudimentary, consisting of membraneous 

 papillae. The anus is small, and is situated between the stigmal plates. 

 The larvae moult twice during the parasitic period. In the third stage 

 they reach their full size, change in colour, and are then ready to leave 

 their hosts ; those living in the skin drop out and pupate in the ground, 

 while those in the various internal organs are either passed out or crawl 

 out in order to pupate. They all feed on the juices of the host, or on 

 the exudation which surrounds them. 



WILLISTON'S KEY TO THE OESTRID LARVAE 



1. Last abdominal segment free, broadly attached ...... 2 



Last abdominal segment (twelfth) retractile within the preced- 

 ing, small and distinctly constricted ........ 7 



2. Larvae with two pairs of chitinized jaws ; that is with two 



outer mouth-hooklets, and two inner, straight, triangular 



points (Horses) (iastrophilus. 



Larvae with two or no mouth booklets .... .... 3 



3. Larvae on the median segments with dorsal, spindle-shaped 



tubercles ; one pair of mouth booklets present ...... 4 



Larvae without such tubercles ; one pair or no mouth booklets 



present ...... . . 5 



4. Antennae broadly separated ; body oval, strongly convex 



above, flat below (Sheep) ... ... Oestrus. 



