DIGESTION 41 



the hinder part of the fore-gut is dilated to form a capacious crop 

 (Fig. 7, B). Here the food is stored before being transmitted, in 

 small quantities at a time, to the mid-gut. It is, however, not only 

 stored but digested, being acted upon always by the salivary secretion, 

 and, at least in the Orthoptera and many Coleoptera, by the digestive 

 juices passed forward from the mid-gut. In the higher Diptera a crop 

 for storing food is again present (Fig. 7, C), but it takes the form of a 

 diverticulum connected to the fore-gut by a narrow duct. The food in 

 this crop is mixed only with saliva, and it suffers very little digestion 

 until it is transferred to the long coiled mid-gut. In the Lepidoptera, 

 and in many Diptera (Culicidae, Tabanidae) (Fig. 7, D), the ingested 

 food goes straight to the mid-gut and is there both stored and di- 

 gested; the 'crop' has all but lost its function as a food reservoir, and 

 is used chiefly (in many Lepidoptera, solely) to receive the air swal- 

 lowed by the insect when it emerges from the pupa (p. 10). In the 

 fleas (Siphonaptera) and sucking lice (Siphunculata) the crop has 

 been lost entirely (Fig. 7, E), and the food is taken straight into a 

 voluminous stomach, where it remains until digestion is complete. 



In some insects the crop seems to have a protective function : fluids 

 with a high osnotic pressure, which is detected by special sense 

 organs, are retained in the crop and passed only very gradually to 

 the mid-gut; in mosquitos, whereas blood is swallowed directly to 

 the mid-gut, syrup is deflected temporarily to the crop and passed on 

 to the mid-gut very slowly. 



In many Diptera the mid-gut consists of several segments charac- 

 terized by differences in the epithelium; the more anterior being 

 concerned probably only with absorption, the more posterior with 

 digestion and absorption. The value of this arrangement when the 

 insect takes in much fluid with its food is obvious; for much of the 

 water and many of the assimilable constituents are removed before 

 the food reaches the digestive juices, and consequently these juices 

 do not suffer unnecessary dilution. In many Heteroptera, this ar- 

 rangement has gone a stage further; there is once again a capacious 

 crop in which the meal is received, but it is a crop composed of mid- 

 gut. In the blood-sucking forms (Cimex and RJwdmus) (Fig. 7, F), 

 the food is not at all digested in this part of the mid-gut, but is 

 merely concentrated by the removal of fluid, and the concentrated 



