49O Studies in the Theory of Descent. 



Never more than eleven segments are present 

 besides the first, which is destitute of eyes ; neither 

 are abdominal legs ever developed. 



The mode of life differs very considerably in the 

 two groups of larvae. Although the Dipterous 

 maggots are not as a rule quite incapable of loco- 

 motion like the grubs of the Hymenoptera (bees, 

 ichneumons), the majority are nevertheless pos- 

 sessed of but little power of movement in the 

 food-substance on which they were deposited as 

 eggs. They do not go in search of food, either 

 because they are parasitic in other insects in the 

 same manner as the ichneumons (Toe/lino), or else 

 they live on decaying animal or vegetable sub- 

 stances or amidst large swarms of their prey, like 

 the larvae of the Syrphida amongst Aphides. 

 They generally undergo pupation in the same 

 place as that which they inhabit as larvae and in- 

 deed in their larval skin which hardens into an 

 oval pupa-case. Some few leave their feeding 

 place and pupate after traversing a short distance 

 (Eristalis). 



As in the case of the Hymenoptera the struc- 

 ture of the larvae can here also be explained by 

 peuliarities in their mode of life. Creatures which 

 live in a mass of food neither require special 

 organs of locomotion nor specially developed 

 organs of sense (eyes). They have no use for 

 the three pairs of jaws since they only 'feed on 

 liquid substances, and the hooks within the mouth 



