XI 



PHYLUM ARTHROPODA 



635 



m 



-^t 



;- .'. ; A 



an 



' 



' | 



V 







i i'ff 



' .-- -;.-; 



mxz 

 _pi 



pj 

 p* 



st 



larva or " maggot " is completely worm-like, without any appen- 



dages, and without any distinct head. In other cases (Lepidoptera, 



&c.) there is a distinct head ; the three thoracic segments have 



three pairs of jointed legs, and the abdominal segments short 



unjointed pro-legs (Fig. 523). In most instances the larvae differ 



widely from the adults in their food and mode of life ; very generally 



the jaws are adapted for biting, even when the mouth of the adult is 



suctorial. After a longer or shorter period passed in this larval 



condition, in which it is usually active and very voracious, the 



young Insect passes into a quiescent or pupa stage, during which 



it remains passive, en- 



closed in a tough integu- R 



ment, while a more or less 



complete reconstruction of 



the organs goes on, result- 



ing in the development of 



all the parts of the perfect 



Insect. The development 



of the new parts takes 



place from certain patches 



of cells, the imagined discs, 



present in the larva. 



In the Diptera the larva 



or maggot is sometimes 



completely devoid of jaws. 



In some Diptera, how- 



ever, the jaws are well 



developed, and there is a 



distinct head. After fre- 



q u e n t moultings the 

 maggot passes either into 

 a quiescent or pupa stage 

 enclosed in a hard skin, or 

 into the stage of an active 

 aquatic pupa, which swims 

 about actively in water 

 and may possess tracheal 

 gills. 



In the Lepidoptera the Iarva3 (" caterpillars ") are worm-like, 

 but with well-developed jaws, three pairs of jointed thoracic legs, 

 and a number of unjointed stumpy abdominal legs (pro-It 1 ;/*). 

 Lepidopterous larvse are often brilliantly coloured, are very active, 

 and feed with voracity, chiefly on leaves and other succulent parts 

 of plants. Eventually they spin a cocoon of a silky substance, 

 enclosed within which, and covered with a tough skin, they pass 

 through a quiescent or pupa condition the condition of the 

 chrysalis (Fig. 523). From the interior of this the imago subse- 



\ 



Fu , 



540. A and B, later stages of the embryo of 

 Hydyophilus with the rudiments of the ex- 

 tremities ; in B the abdominal appendages are 

 visible, a. anus ; an. antenna ; g, rudiment of the 

 ventral nerve-chain ; m. mouth ; mil. mandible ; 

 mxi, first maxilla ; mxz, second maxilla ; pi, $2, 

 Pi, thoracic legs ; p, p^, p-, PS, rudiments of the 

 appendages of the first, second, fourth, and sixth 

 abdominal appendages ; st. stigmata ; vk, prosto- 

 miuin. (From Lang, after Heider.) 



