V] THE ALIMENTARY AND EXCRETORY SYSTEMS 113 



process of digestion, in length rather than in width. Thus it may 

 extend sometimes only from the fourth segment to the sixth, while 

 at other times it is somewhat wider, and reaches from the beginning 

 of the second to the end of the seventh segment. It is always 

 widest at its anterior end, where it receives the invaginated gizzard. 

 For the rest of its length it is practically cylindrical. The physiology 

 of digestion has not been closely studied, as in the larva. We know, 

 however, that the imago feeds more often and in smaller quantities 

 than the larva does. Hence we may conclude that the processes 

 of digestion are not marked by such violent changes as in the larva, 

 though they are of the same type. The peritrophic menlbrane is 

 evidently not thrown off along the whole length of the mid-gut after 

 each meal, since the faecal pellets are comparatively short and 

 numerous. 



The Hind-gut. 

 1. Small Intestine or Ileum. 



The mid-gut contracts sharply at its posterior end, to open by 

 a narrow orifice into the anterior portion of the hind-gut, known 

 as the small intestine. The extreme posterior end is closed by a 

 strong sphincter muscle, which effectively prevents the passage of 

 any impurities from the hind-gut back into the mid-gut. Imme- 

 diately below this muscle lies the zone of entry of the Malpighian 

 tubules (p. 117). These tubes do not enter separately, but unite into 

 bundles of five or six, forming a common conduit. These conduits 

 become excessively narrowed, and open into the intestine by a ring 

 of minute pores. Below this zone, another sphincter muscle is 

 strongly developed. 



In the larva, the anterior portion of the small intestine is narrow, 

 and may be called the short intestine. The posterior part is dilated 

 into a spherical bulb, called the pre-rectal ampulla [145]. The differences 

 between these two portions appear to be only those of degree, and not 

 of kind. In each case we note the return, for the most part, to the 

 familiar formation already described for the fore-gut, and consisting 

 of the usual four layers of cuticle, flat epithelium, circular muscle 

 tunic and weaker longitudinal muscle tunic, from within outwards. 

 The latter tunic is arranged in six bands. In the short intestine, 

 however, nearly the whole of the ventral half of the epithelium is 



T. D.-F. 8 



