494 



INVERTEBRATE MORPHOLOGY. 



oesophagus, wliose posterior region is frequently dilated into a 

 crop (Fig. 227, c) which in some Beetles is lined with chitinous 

 teeth or bars and whose walls are muscular, the apparatus 

 probably serving for a further mastication of the food. The 

 mid-gut which succeeds the crop is usually dilated into 

 a stomach (s), lined in some cases by glandular cells, or, 

 in others, having opening into it numerous glandular diver- 

 ticula, the so-called liver-pouches. The hind-gut, like the 

 fore-gut of ectodermal origin, has opening into its anterior 

 extremity the Malpighian tubules {mv), which vary consider- 

 ably in number, amounting to nearly one hundred in some 

 Hymenopterans, though more usually limited to from four to 

 eight. They are excretory in function, and are apparently the 



■--li. A ■■--:;. J B 



Fig. 338. — Different Akeangbmbnts of the Nebtotts System in Insects 

 (from Gegenbauk). A, Termes ; B, Bytiscui ; 0, ally. 



only excretory organs which occur. The anus is situated at 

 the extremity of the body, and in close proximity to it odor- 

 iferous glands frequently open into the hind-gut, serving as 

 organs of defence. In some cases they secrete an acrid fluid 

 which, as in the Bombardier beetle {BracTiinus), can be ex- 

 pelled with almost explosive force. 



The nervous system in forms where it shows the least 

 amount of modification (Fig. 228, A) consists (1) of a supra- 

 oesophageal mass composed apparently of three pairs of 

 ganglia and supplying the eyes and the antennre ; (2) of a sub- 

 oesophageal mass composed also of three pairs of gano-lia 

 supplying the segments indicated by the mandibles, the 



