6 The Structure and Special Physiology of Insects 



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



compound eye, 

 antennae^ 

 prothorax^ ' 



S 



labial 

 palpi 



proboscis' 



\ \metathoraz 

 * \ mesothorax 



.aur/t, 



tarsal segments 



FlG. 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 



7" r , may be wholly lost, as is true of the mandibles 

 FIG. 8. Dorsal aspect of head * *, 



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



nuta, female, showing mouth- k ears a i so t h e i arge compound eves and the 



parts. lb., labrum, removed; ,, . . ... ... 



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

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

 bium; st., stipes of maxilla; ., . ri i_-v i .*. j j 

 nixp., palpus of maxilla; ant., thr e g pairs of legs, 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 



