30 



The Larva of CJiiroiwinus 



a minute projection in the floor of tlie nioutli (lingua). 

 We should naturally expect to find the brain in the 

 head, but in the blood-worm it has been retracted into 

 the segment next behind (prothorax). In the fresh- 

 hatched larva, however, it occupies its normal position in 

 the head. A few words of explanation may be given 

 here, though the subject is ^more fully discussed in 

 chapter iv. The larval head is small in Clilronomus 

 dorsalis and other blood-worms, as in many other insects 

 which feed upon dead organic matter. Their food is 

 plentiful and ready to hand, so that highly developed 



Fio. 19. — Median section tliruugh larvul lieail. ws, oesophagus. <?, its diverti- 

 culum, dv, dorsul vessel, fg, frontal ganglion. I, labrum. mt, mentum. 6m, 

 submentum. m/n, muscles of mandibles, &c. m'm', muscles which hold the 

 oesophagus in ^ilace. .s^, salivary duct. 



sense-organs are not required in this stage. But the 

 head of the fly, which is larger, much more complex, and 

 quite different in shape, has to be formed within the 

 body of the larva. It is, we may remark, a very wide- 

 spread error to suppose that the head and other organs 

 of the imago form during the pupal stage ; their develop- 

 ment is nearly always far advanced when the pupal stage 

 begins. The imaginal head is moulded out of folds of the 



