The Structure and Special Physiology of Insects 9 



long, flat, slender, and saw-toothed, as with the scorpion-flies, or needle-like, 

 as in all the sucking bugs, or reduced to mere rudiments or wholly lacking, 

 as in the moths and butterflies. Similarly with the other parts. But by 

 careful study of the comparative anatomy of the mouth structure, and par- 

 ticularly by tracing its development in typical species representing the 

 various types of biting, sucking, and lapping mouths, all the various kinds of 

 mouth structure can be compared and the homologies or structural cor- 

 respondences of the component parts determined. Figs. 8 to 16 illustrate 



FIG. 13. FIG. 14. FIG. 15. 



FIG. 13. Seventeen-year cicada, Cicada septendecim, sucking sap from twig. (After 



Quaintance; natural size.) 

 FIG. 14. Section of twig of Carolina poplar showing beak of cicada in position when 



sucking. (After Quaintance; much enlarged.) 

 FIG. 15. Mouth-parts, much enlarged, of net-winged midge, Bibicocephala doanei, 



female, md., mandible; mx., maxilla; mx.l., maxillary lobe; mx.p., maxillary 



palpus; It,, labium; hyp., hypopharynx; pg., paraglossa of labium; l.ep., labrum 



and epipharynx. 



examples of different mouth structures, with the corresponding parts similarly 

 lettered. 



The most conspicuous structural characteristic of insects is their poses- 

 sion of wings. And the wings undoubtedly account for much of the success 

 of the insect type. Insects are the dominant animal group of this age, as 

 far as number of species constitutes dominance, their total largely sur- 

 passing that of the species of all the other kinds of living animals. Flight 

 is an extremely effective mode of locomotion, being swift, unimpeded by 

 obstacles, and hence direct and distance-saving, and an animal in flight 

 is safe from most of its enemies. The wings of insects are not modified true 

 appendages of the body, but arise as simple sac-like expansions (Fig. 17) 

 of the body-wall or skin much flattened and supported by a framework of 



