6 The Structure and Special Physiology of Insects 



the segments of the jibdomcn retain their independence and are more or 



compound eye, 

 antennae 



prothorax^ '■ 



labial 

 palpi 



proboscis' 



tarsal segments 



Fig. 7. — Body of the monarch butterfly, Anosia plexippus, with scales removed to show 

 external parts. (Much enlarged.) 



less similar, thus preserving a generalized or ancestral condition. On the 

 head are usually four pairs of jointed appendages (Fig. 8), viz., the 



antenna; and three pairs of mouth-parts, 

 known as mandibles, maxilla;, and labium or 

 under-lip. Of these the mandibles in most 

 cases are only one-segmented, while the tw(3 

 members of the labial pair have fused along 

 their inner edges to form the single lip-like 

 labium. The so-called upper lip or labrum, 

 closing the mouth above, is simply a fold of 

 the skin, and is not homologous, as a true 

 appendage or pair of appendages, with the 

 other mouth-parts. In some insects with highly 

 modified mouth structure certain of the p;irts 

 may be wholly lost, as is true of the mandibles 

 in the case of all the butterflies. The head 

 bears also the large compound eyes and the 

 smaller simple eyes or ocelli (for an account of 

 Attached to the thorax are 



Fig. 8. — Dorsal aspect of head 

 of dobson-fly, Corydolh cor- 

 ttula, female, showing mouth- 

 parts, lb., labrum, removed; 

 md., mandible; nix., maxilla; 



/»"., labium; gl, glo^sje of la- the eyes see p. •?o). 



bium; si., stipes of maxilla; .> . r 1 1 • 1 • • , , 



mxp., palpus of ma.xilla; ant., three pairs of legs, which are jointed appendages, 

 antenna. homologous in origin and fundamental struc- 



ture with the mouth-parls and antennae, and two pairs of wings (one or 



