408 ARTHROPODA 



(fig. 447) is a long tube coiled like a watch spring beneath the head. It con- 

 sists of two long grooved maxillary galea firmly united by their edges. The 

 maxillary palpi are well developed in the moths; elsewhere they show all stages 

 of reduction to complete disappearance. Labium and labrum are reduced to 

 small triangular plates at the base of the proboscis, the labium bearing a pair of 

 hairy palpi (pi). The mandibles are represented by small plates or bunches of 

 hair. These conditions gain in interest when we remember that in the larva 

 the mandibles are strong biting organs, while the maxillae are small hooks, and 

 the labium is better developed only in those parts connected with the silk glands, 

 a beautiful example of relations of structure to life conditions. 



In contrast to the other regions, the abdomen lacks appendages in the adults. 

 Only in the Thysanura are small lobes present, behind and in the same line with 

 the thoracic fee't, which may be abdominal feet. Apparently, too, the append- 

 ages of the last segment, the stylets and cerci, are modified limbs, but the parts 

 (qonapophyses) used in copulation and oviposition are different in character. 

 False feet, or pro-legs, occur on the abdomen of the larvas of the Lepidoptera and 

 the Tenthredinidas, but since these are fleshy un jointed processes, it is doubtful 

 whether these are true abdominal limbs, or are structures independently 

 acquired. 



Besides ventral appendages the insects usually have two pairs of dorsal 

 outgrowths upon the meso- and metathorax, the wings. They are lateral 

 folds of the chitinous coat of the notum and contain on their interior exten- 

 sions of the blood sinuses and of the tracheae, which are protected by thick- 

 enings of the chitin, causing the network of 'veins' or 'nervures' in the wing. 

 Both wings may be elastic, flexible, and adapted for flight, or the hinder 

 pair may alone partake of this character (true wings or alec}, while the 

 first pair may be thick and parchment-like wing covers, or elytra, under 

 which the true wings are concealed when at rest. When only the base of 

 the wing is thus thickened hemelytra result. Between the bases of the 

 anterior wings is frequently a chitinous plate, the scuteUum, between the 

 hinder wings a similar postscutellwn. In many insects one pair of wings 

 is lacking, the anterior pair being retained in the Diptera (fig. 486), the 

 posterior in the Strepsiptera (fig. 469). The entire absence of wings may 

 occur from two causes; wings have apparently never been developed in 

 some (primary lack of wings of the Apterygota), while there are others in 

 which wings once' present have been lost, because nearly related forms 

 bugs, male cockroaches, sexual ants and termites are winged (figs. 

 464, 482, 483). The prothorax of all recent insects is wingless, but some 

 Archiptera of the coal period had wing rudiments on this somite. 



As a result of differences in food the alimentary canal (figs. 448, 449) 

 varies greatly. The ectodermal stomodaeum begins with a pharynx, which 

 in the sucking insects is a sucking apparatus with radial muscles. The 

 oesophagus, which follows, may be widened to a crop (ingluvies), or it may 

 have a cascal outgrowth which in the butterflies and flies may take the 

 shape of a stalked vesicle (falsely 'sucking stomach'). Also ectodermal 



