42 INSECT PHYSIOLOGY 



product passed on for digestion to the long narrow intestine. 



In these insects the fluid that is absorbed passes into the blood and 

 is then excreted by the Malpighian tubes; but an advance on this 

 mechanism has been developed by many Homoptera (Cicadoidea, 

 Coccoidea, &c). These feed on the juices of plants, and therefore 

 receive a great excess both of water and of sugars. But instead of the 

 superfluous fluid being taken into the blood and then eliminated by 

 the Malpighian tubules in the manner just described, a dilated loop 

 of the mid-gut at its commencement, with very delicate walls, is 

 intimately associated with the posterior end of the mid-gut. In this 

 way the fluid contents of the food can be absorbed at the beginning 

 of the mid-gut and transferred directly to the rectum without the 

 haemolymph or the general content of the mid-gut being unneces- 

 sarily diluted. There are many modifications of this so-called 'filter 

 complex' and much remains to be learned about its physiology. The 

 excess fluids are discharged to the exterior as honey-dew, &c. (p. 

 52). Many modifications of this arrangement have been described 

 (Fig. 7, G). 



Another special modification of the gut is met with in the larvae 

 of Hymenoptera-Apocrita, and in the larvae of Myrmeleon and other 

 Neuroptera (Fig. 7, H). In these the mid-gut remains unconnected 

 with the hind-gut until the time of pupation. Consequently, the 

 stomach becomes enormously distended with the undigested residue 

 of the food, which is not evacuated until just before pupation. 



The proventriculus 



In nearly all insects there are special structures where the fore-gut 

 and the mid-gut join. There is nearly always a muscular sphincter by 

 which the contents of the two regions can be kept separate; and there 

 is often a discrete organ known as the proventriculus. The proventri- 

 culus is a different morphological entity in different groups, some- 

 times being composed solely of fore-gut, while sometimes both fore- 

 gut and mid-gut contribute to it. Its function also varies. In the 

 Blattidae it is a powerful muscular organ or gizzard provided with 

 rows of massive teeth, which seize and triturate the solid particles of 

 food in the crop. In the bee and other Hymenoptera the proventri- 

 culus is termed the 'honey stopper'. It forms an elongated plug with 



