494 



INVERTEBRATE MORPHOLOGY. 



oesophagus, whose posterior region is frequently dilated into a 

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

 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 ectoderrnal origin, has opening into its anterior 

 extremity the Malpighian tubules (mv). which vary consider- 

 ably in number, amounting to nearly one hundred in some 

 Hymeuopteraus, though more usually limited to from four to 

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



o 



\ 



A B \ // 



FIG. 228. DIFFERENT ARRANGEMENTS OF THE NERVOUS SYSTEM IN INSECTS 

 (from GEGENBAUR). A, Termes; B, Dytiscus; C, a fly. 



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 (BracUnus), 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- 

 cesophageal mass composed apparently of three pairs of 

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

 cesophageal mass composed also of three pairs of ganglia 

 supplying the segments indicated by the mandibles, the 



