INSECTA 385 



labium, we have a character which is found in the maxillipeds of 

 certain Crustacea. Fig. 288 indicates the similarity between the in- 

 sectan and crustacean mouth parts. Such an attempt at a comparison 

 is only possible with the more generalized mouth appendages of the 

 Insecta. In some of the more speciahzed orders the parts are so 

 modified as to leave no evidence of their primitive structure. Such 

 modifications will be deah with in connection with the different orders. 



The thorax is separated from the head by a flexible neck region 

 usually containing cervical sclerites, which, however, have not any 

 segmental value. It consists of three segments — the prothorax, which 

 carries a pair of legs but no wings, the mesothorax and the meta- 

 thorax, which each bear a pair of legs and, typically, wings. The legs 

 are made up of five main segments, the coxa and trochanter (both of 

 which are small), the femur and tibia (which form the greater part of 

 the limb), and the tarsus (which is usually further subdivided into a 

 number of joints, and ends in a pair of claws with a cushion between 

 them called the pulmllus). Of the many adaptations exhibited by the 

 legs of insects the jumping type found in grasshoppers, the digging 

 type in the mole-cricket Gryllotalpa, the swimming type in the 

 water beetles like Dytiscus, the prehensile type in the fore legs of the 

 praying insect Mantis may be mentioned, in addition to the ordinary 

 running type as seen in a cockroach. Modifications for the production 

 or reception of sound as in the Orthoptera and for the collection of 

 food (the combs and pollen baskets of bees) are also familiar. 



The wings of an insect are thin folds of the skin flattened in a 

 horizontal plane, arising from the region between the tergum and 

 pleuron. A section of a wing bud shows two layers of hypodermis, 

 the cells of which are greatly elongated (Fig. 301). Into the blood 

 space between the layers grow tracheae, and when in a later stage the 

 two layers of hypodermis come together and the basement mem- 

 branes meet and fuse, spaces are left round the tracheae which form 

 the future longitudinal wing veins. These spaces contain blood and 

 sometimes a nerve fibre during development. The cuticle round the 

 veins is much thicker than in the general wing membrane, so that the 

 veins are actually a strengthening framework for the wing. The 

 number and arrangement of the veins is highly characteristic of the 

 diff"erent groups. Though the majority of insects possess wings there 

 are important orders which are wingless. Some such as those to 

 which the fleas and lice belong are secondarily so, because of their 

 parasitic habit. Others, however, constituting the large division 

 Apterygota, are primitively wingless, and these, both on morpho- 

 logical and palaeontological evidence, must be regarded as the most 

 ancient types known. The variations in form, consistency, and size 

 of the wings are briefly dealt with under the different orders. 



BI 35 



