230 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



in the various orders or main subdivisions of the insects. In the Dragon- 



Mav-tlirs. Termites and other insects commonly grouped together as 



ra tlu- two pairs of wings are membranous and much alike. In 



IKmenoptera (Bees, Wasps, Ants) both pairs of wings are again 



membranous, but the hinder wings are smaller and are attached to the 



- by numerous little hooks which project from the front edge 



of the hind-win- and hook over the hind edge of the fore-wing. Fore- 



and hind-wing on each side thus act as if they were a single continuous 



structure. In the Diptera (ordinary flies, Mosquitos, Midges) the fore- 



win-s are membranous while the hind-wings are converted into curious 



dub-shaped organs of unknown function called halteres. 



In the Lepidoptera (Butterflies and Moths) the numerous little bristles 

 scattered over the surface of the wing have taken the form of overlapping 

 which the colours of the wings are due. In the Orthoptera 

 !v roach, Locust, Grasshopper, Earwig) the front wings have lost 

 their function in connexion with flight : they have become thick and 

 horny and serve as wing-covers to protect the delicate membranous 

 hind-wings when these are not in use. In the Hemiptera the two pairs 

 of win^s may be thin and membranous (Aphis Green fly) but more 

 usually the fore-wings are thickened and protective as in the Orthoptera, 

 only in this case the thickening of the wing is often more conspicuous 

 in its basal portion (many Bugs). The protective modification of the 

 lorc-win-s finds its greatest development in the Coleoptera or Beetles 

 where' they are greatly thickened and are known as elytra. 



\Vin-s are thoroughly characteristic of the group of Insects. Among 

 those which are without them we have to distinguish between those in 

 which the loss of wings is probably to be regarded as a secondary develop- 

 ment such as insects inhabiting small islands in the ocean and insects 

 that have adopted a parasitic mode of life and those which probably 

 represent a primitive wingless stage of insect evolution (Aptera : such as 



>nn and Machilis). 



The wings of insects arise in the embryo as flat flaps of skin which 

 grow out from the surface but much more frequently arise in the 

 "I deep pockets of the skin, the " imaginal disc " corresponding 

 h win- having sunk deep below the surface. As regards their 

 ury origin we are still pretty much in the dark, but there is 

 n to suspect that the extensions of the body-surface which 

 function as wings were primitively respiratory in function. In the 

 of certain insects (May-flies) there are present flat plate- 

 like gills on the abdominal segments which present a striking resemblance 

 ire to wings and there is a tendency to revert to the view, 



