6 The Structure and Special Physiology of Insects 



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



\\melathoraz 

 v\ mesothorax 



compound eye, 



antennse.... 



\ I 

 prothorax *<./ 



labial 

 palpi 



proboscis'' 



.nurp. 



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 



antennae and three pairs of mouth-parts, 

 known as mandibles, maxillae, and labium or 

 under-lip. Of these the mandibles in most 

 cases are only one-segmented, while the two 

 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 parts 



FIG. 8,-Dorsal aspect of head ma y be wholl y lost > as is true of the mandibles 

 of dobson-fly, Corydalis cor- in the case of all the butterflies. The head 



nuta, female, showing mouth- bears also the large compound eyes and the 

 parts, lo., labrum, removed; . ,. 



md., mandible; mx., maxilla; smaller simple eyes or ocelli (for an account of 



/., labium; gl, glossae of la- the eyes see p. 30). Attached to the thorax are 



bium; St., stipes of maxilla; Al . r , , . , . . , , 



mxp., palpus of maxilla; ant., three P airs of le g s > which are jointed appendages, 



antenna. homologous in origin and fundamental struc- 

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



