THE INTERNAL STRUCTURE OF ANTS. 35 



modification. This is the pumping stomach proper. It is succeeded by 

 a slender, thin- walled tube, the cylindrical section (/?), opening behind 

 into the much more voluminous stomach on the summit of a knob, 

 which is also valvular in structure (i). At this point the chitinous lining 

 of the alimentary tract stops abruptly. The walls of the proventriculus, 

 especially of its bulb, are furnished with powerful transverse and feebler 

 longitudinal muscles. 



The function of the proventriculus as a pump has been explained 

 by Emery. It is clear from the shape of the chitinous folds in the bulb 

 and the arrangement of the musculature that the contraction of the 

 latter must bring the folds close together and occlude the lumen, whereas 

 the relaxation of the muscles permits the chitinous folds to flatten out 

 through their own elasticity and thus enlarge the cavity and suck the 

 liquid back out of the crop. Hence the organ functions like a rubber 

 bulb with a tube and an appropriately constructed valve at each end. 

 When the bulb is squeezed its liquid contents are forced into one tube, 

 and when it is permitted to expand, it draws the liquid out of the other 

 tube. The proventriculus has an important function, not only in 

 passing the liquid food back from the crop to the true stomach, but also 

 in filling the crop in the first place. 



The proventriculus of Caitiponotus may be regarded as representing 

 a structure from which we can pass on the one hand through greater 

 simplification to the Myrmicine and Ponerine proventriculus, and on 

 the other through greater complication to that of the other Camponotinje 

 (Plagiolepis, Prcnolcpis, etc.) and Doli'choderinge. This complication 

 consists, in great part, in a shortening of the calyx and a spreading 

 and recurving of its lips till they form a bell-shaped structure more or 

 less completely enclosing the remainder of the proventriculus. Extreme 

 forms of this kind are seen in Iridoinyrme.r and Technomyrmex ( Fig. 

 16, E, F}. In these ants it is possible to see how the proventriculus 

 may play an important role in regurgitation as well as in ingurgitation, 

 for the contraction of the walls of the crop, especially of the ring- 

 muscles at the posterior end, and the pressure of its liquid contents 

 must tend to close the openings between the sepals, thus preventing 

 the liquid from moving backward and determining its flow in the oppo- 

 site direction. As the musculature of the crop is poorly developed, 

 some authors, like Janet, regard the pharynx as the organ which by its 

 peristaltic contractions probably initiates regurgitation and may even 

 be of great importance in filling the crop during ingurgitation. 



All of the above-described regions of the alimentary tract arise in 

 the embryo as a tubular infolding of the outside skin, or ectoderm, the 

 so-called stomodseum. This is indicated in the adult by the almost 



