VIII AETHEOPODA 287 



wings ; and that in the most modified Insecta a worm-like larval 

 stage is found. 



Now we have proceeded on the assumption, which all the clearest 

 cases seem to warrant, that the larva always represents, with more or 

 less physiological modification, an ancestral condition of the stock. 

 Why, therefore, should the lowest branch of the Insecta fail to show 

 this larval stage more clearly than the highest and most modified 

 branch ? Is it possible that a larval stage may be secondarily inter- 

 calated in a life-history in which the young were originally hatched 

 with all the adult characters ? Such considerations force us to look 

 at Insect larvae a little more closely from another point of view. 



Now we find that the caterpillar form of larva, with biting mouth- 

 parts and abdominal appendages on some of the segments, crops up 

 not only in Lepidoptera, but also in the more primitive Hymenoptera ; 

 but in the former case the appendages are borne on segments 2, .3, 4, 5, 

 and 10 of the abdomen, and in the latter case on segments 3, 4, 5, 6, 

 7, 8, and 10. Again an adult form with biting mouth-parts and rudi- 

 mentary abdominal appendages, which, as a matter of fact, are used 

 to assist in locomotion, is found amongst the Thysanura, as, for 

 instance, in Machilis. We conclude, therefore, that the caterpillar, 

 or eruciform type of larva, represents a Thysanuran ancestor, and 

 the reason why the Thysanura do not exhibit this phase in the 

 earlier portion of their life-history is that it represents their adult 

 condition. 



But just as the primitive type of Echinoderm larva, found amongst 

 Asteroidea, has undergone modification and reduction amongst the 

 more modified groups of Ophiuroidea, Echinoidea, and Holothuroidea, 

 so the eruciform type has undergone modification and reduction in 

 other types of higher insects, till it becomes, in extreme cases, a legless 

 " grub," as in Muscidae. Where the mother, in consequence of her 

 improved powers of flight and her developed senses and instincts, is 

 able to provide her offspring with a store of nourishment which does 

 not require much mastication, and which ensures there shall be 

 demand for little or no active exertion on the part of the offspring to ' 

 gain food, then not only wiU abdominal appendages disappear, but 

 also the thoracic appendages, and even the gnathites or jaws. In fact, 

 we then have influences acting on the larvae similar to those which 

 have produced legless and senseless j)arasites from 'adult forms in 

 other groups of the animal kingdom. 



It must be admitted that this explanation will not fit the case of 

 Coleopterous larvae, many of which are quite active, but which do 

 not develop abdominal appendages. There must be some independent 

 reason for the suppression of these appendages in the larva which 

 still may appear in the embryo {Melolontha), but which may be sup- 

 pressed even there (Donacia). 



The lower winged insects, often termed from the absence of a 

 pupal stage in their development Hemimetabola, pass through the 

 stage corresponding to the caterpillar, within the egg membrane ; as 



