THE BIOLOGY OF INSECTS 



body-covering ; they belong to that great race ^ of animals 

 among whose members the living skin (Fig. i, e) forms out- 

 side itself a more or less firm and resistant cuticle, composed 

 of a horny substance called chitin (the chemical composition 

 of which has been represented by the formula C30H5QO1QN4), 

 and thickened segmentally in agreement with the marked 

 segmentation of the body and limbs so as to build up a 

 jointed exoskeleton. This coat of mail, as it may be some- 

 what fancifully termed, affords the living insect much 

 protection from its enemies and also enables it to achieve 



great precision and rapidity of 

 movement, since the muscles 

 of the trunk and limbs are 

 attached internally to the hard 

 firm parts of the exoskeleton, 

 which being united by tracts 

 of flexible cuticle, move readily 

 on one another. Further, it is 

 noteworthy that in this great 

 group of animals the muscle- 

 fibres when viewed with the 

 microscope, show the same kind 

 of cross-striping that charac- 

 terises the body-muscles of 

 vertebrates. 



Other classes of animals be- 

 sides insects are built on the 

 general plan just described — spiders, centipedes, millipedes, 

 lobsters and crabs for example. But while most of these 

 have many pairs of jointed limbs adapted for crawling or 

 swimming, insects generally have the legs reduced in 

 number to six, and the great majority of them display a 

 highly characteristic feature in the presence of two pairs 

 of flattened outgrowths arising from the dorso-lateral region 

 of two adjacent, forwardly situated body-segments (Fig. 2). 

 These outgrowths, jointed on to the body and capable of 

 depression and elevation by the action of suitable muscles 



^ The Arthropoda. See Classification, Chapter XI. 



Fig. I . — Section through cuticle 

 (c) and skin or epidermis (e) 

 of the leg-base of a Bluebottle 

 (Calliphora) just after casting 

 the pupal cuticle, h, sensory 

 hair ; n, nerve-cell. X 300. 



