334 INTRODUCTION TO ZOOLOGY ciiAi'. 



feeding on the decaying matter in it. The appearance of the 

 larva is shown in Fig. 253. It has a greyish- white soft body, 

 about § of an inch long, with seven pairs of short processes 

 underneath, armed with fine hooks by means of whicli it 

 crawls along. The head is also soft and rounded, bearing 

 the mouth on its under side ; just above this are two very 

 small processes which appear to be sensory. The mouth 

 leads into a peculiar throat which can be dilated, and then 

 acts as a suction pump drawing in the food.^ 



The tail is, however, quite the most conspicuously remark- 

 able structure possessed by the larva, and this organ has 

 earned for it the name of the Rat-tailed Maggot. It can 

 be held at any angle to the body, and is capable of great 

 variation in length, so that within limits, whatever the depth 

 of the water, the tail is adjusted so that its tip just reaches 

 the surface ; the greatest length attainable seems to be about 

 4^ or 5 inches, and this is a good deal when 

 the size of the larva is considered. The 

 variation in length is due "to the fact that the 

 terminal, dark-coloured, thinner portion of the 

 tail can be withdrawn into the wider tubular 

 part below, and the latter can itself be also 

 contracted, the whole surface being drawn 

 into transverse wrinkles, until it is only about 

 half an inch long. The tail has at its tip some 

 -little recurved bristles which spread out on 

 the surface film of the water, exposing to the 

 air the spiracles connected with the two air- 

 tubes that run side by side down the tail to 

 join the main tracheae of the body (Fig. 254). 

 When full grown the maggot usually leaves 

 the water, and enters the earth 

 where this is damp and loose, >;^ffl5"|*t 

 — =-F^™^ and there pupates inside its (^~'"*»W*-' 

 Fig. 254. — The larval skin. The body and tail ^y 

 upper end of shorten, the skin turns brownish ^ 



the Tail of a ^nd hard, and the body within Fig. 255.— The 

 y • Tjji(jergoes changes similar to 1'"^'' °^ ^ 



S, Spiracles. • ^u "Di a T • i,<- Drone-fly. 



those in the Islow-iiy. In eight 



1 See the account based on J. J. Wilklnsou's investigations given in Natural 

 History of Aquatic Insects, by Professor Miall. 



