INSECTA 429 



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 wines 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. 320). 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 

 difi^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. 



Among many orders of insects, there has developed a tendency for 

 the two pairs of wings to act as one. This is accomplished by various 

 devices which couple the fore and hind wings together, on each side. 

 In the scorpion flies, e.g. Panorpa, bristles project back from the 

 posterior or jugal lobe of the fore wing to overlie the anterior border 

 of the hind wing. Corresponding bristles to these constituting the 

 frenulum, project forwards from the anterior border of the hind wing, 

 and overlie the posterior border of the fore wing. In most Lepidop- 

 tera, frenular bristles of the hind wing are held in position by a group 

 of curved setae forming a retinaculum on the fore wing. The two 

 wings of a side, in Hymenoptera are coupled by a row of hooks — 

 the hamuli — on the anterior border of the hind wing, engaging in 

 a fold of the posterior border of the fore wing. 



In other orders, we find one pair of wings diverted to uses other 

 than flight, the latter operation, then, being dependent on one pair 

 of wings. The fore wings, for instance, of Orthoptera and of Dermap- 

 tera, are protective to the more delicate folding flight wings, behind 

 them. The elytra, or fore wings of beetles, are similarly protective, 

 and are held passively extended while the second pair of wings propel 

 the animal through the air. In the males of Strepsiptera, the anterior, 



