332 THE BIOLOGY OF INSECTS 



that not only in the organs of feeding and locomotion, but 

 in the ways of their development and growth have insects 

 as a class undergone much change in the course of ages, 

 even as most individual insects undergo profound changes 

 in their usually brief lives. Among the Apterygota the 

 young resemble generally their parents and live under the 

 same conditions ; there is no marked transformation 

 in the life-history. The same absence of any marked 

 change of form is an obvious character in many orders of 

 the Exopterygota — the Dermaptera, Orthoptera, Isoptera, 

 Mallophaga, Anoplura, and many Hemiptera. Such 

 ** ametabolous " condition, as we have seen, must be re- 

 garded as a primitive character among insects. In certain 

 families of Hemiptera, such as the Cicadidae and the 

 Coccidae, the young differ markedly from their parents, 

 and may be described as larvae, and these passing through 

 an '' incomplete " transformation are often described as 

 ** hemimetabolous." In many of them, as among the 

 Thysanoptera, the insect is predominantly passive before 

 its final moult. These facts combine to emphasise a diver- 

 gence between the adult and immature stages, derived from 

 a primitive condition in which both are alike. This inter- 

 pretation by increasing divergence is especially suggested 

 by the Ufe-history of the three orders Plecoptera, Epheme- 

 roptera, and Odonata, whose members, aerial when adult, 

 have a long larval life under water. In the Plecoptera, 

 the young aquatic stonefly-grub is not unlike its parent. 

 Differing from the latter in its place of abode, it does not 

 differ markedly in form, though it has special thoracic 

 gills for breathing the dissolved air. Among the Odonata 

 there is much greater divergence, and some of the 

 specialisations of dragon-fly larvae have already been men- 

 tioned (pp. 44, 277, 313). While the transformation of the 

 labium into the specialised hinged and hooked " mask," 

 which enables the lurking larva to stalk its prey, is common 

 to the whole order, the adaptations for breathing dissolved 

 air differ markedly in the two sub-orders of dragon-flies. 

 It may be inferred from this that the divergence in jaw- 



