FEEDING AND BREATHING 25 



narrow tubular gullet traverses the thorax and then, in the 

 abdomen, opens out into a capacious ovoid crop in which a 

 quantity of material can be stored awaiting digestion. In 

 the bee the crop is similar in form and arrangement but 

 relatively smaller (Fig. 10, cr.). A butterfly has the front 

 end of the gullet expanded into a small spherical bulb 

 situated in the head, this by the alternate expansion and 

 contraction of its wall, induced by the action of suitably 

 arranged muscles, sucks liquid nectar into its cavity and then 

 propels it along the gullet into the crop. And the butter- 

 fly's crop is not simply an expanded region in the course of 

 the fore-gut, it is a sub-globular sac forming a lateral out- 

 growth at the hinder end of the gullet, so that the liquid 

 food may accumulate in it, and pass on later to the further 

 regions of the digestive tube. In a two-winged fly of the 

 common housefly or bluebottle type, the gullet is pro- 

 longed backwards far beyond the front end of the abdomen, 

 widening into the sub-globular or ovoid crop which, having 

 no other outlet, serves in this case also as a reservoir for 

 liquid food. 



The proventriculus (Fig. 10, pv) is the third or final 

 section of the fore-gut. In a biting insect such as a cock- 

 roach or beetle, this forms a short region of the digestive 

 tract, sub-globular or hemispherical in shape, with the 

 muscular coat of its wall very thick, and its internal chitinous 

 lining raised into strong tooth-hke ridges which project 

 into the cavity. Contraction of the muscle fibres tends to 

 bring these teeth together in the mid-cavity of the organ 

 which is frequently spoken of as a '* gizzard," under the 

 assumption that its function is the crushing of solid food- 

 material. Probably, however, the action of the proventi- 

 culus is rather that of a strainer, preventing the passage of 

 the food into the middle region of the food-canal until its 

 treatment in the fore-gut has been brought to completion. 

 In very many insects the hindmost region of the proventri- 

 culus projects as a comparatively narrow tube into the front 

 end of the stomach or mid-gut. This arrangement may be 

 well seen in that familiar insect the hive-bee (Figs. 10, 11), 



