BIOLOGY AND STRUCTURE OF HYDKOPHILUS CARABOIDES 641 



consists of the successive action of the suctorial and pharyngeal 

 apparatus in the order described above. 



Generall}^ speaking, one may say that in the head of 

 H. caraboides the fore-gut is organized as a forcing 

 pump ; during the suction the part of sucker of the pump 

 is played by the upper wall of the suctorial apparatus, 

 which is lifted up by the levators : simultaneously the pharyn- 

 geal apparatus contracts its sphincters and acts as the valve 

 of the pump, preventing the food already swallowed into the 

 stomach from being sucked liack again. 



At the next act — the forcing — the action of the different 

 parts of the apparatus change. The pharyngeal apparatus acts 

 as a sucking contrivance, and the proper suctorial apparatus 

 comes to a state of rest and, pressing on the liquid it contains, 

 produces a forcing action. This liquid cannot re-enter the 

 body of the prey, as the latter, being worked into a lump, is 

 tightly pressed towards the oral aperture and is, moreover, 

 firmly squeezed with the mandibles. 



Consequently not only liquid parts of the prey are swallowed, 

 but also solid portions of its body, as for instance bits of 

 tracheae, bits of the chitinous cuticula, &c. I have found 

 similar remnants in the rectal sac of the larva of H. cara- 

 boides (PI. 27, fig. 1, ch), a fact which would be impossible in 

 the case of typical suctorial insects, as for instance the larva 

 of Dytiscus. 



In the latter the food enters the suctorial apparatus by 

 canals in the mandibles which remain perfectly immovable 

 during the suction of food ; as the latter is submitted to 

 a chemical treatment by ferments only, a mechanical action on 

 the food-stuff is absolutely excluded. 



It is interesting that the general idea of a sucking and forcing 

 apparatus (consisting of two contrivances — the suctorial 

 apparatus proper and the pharyngeal apparatus) is repeated 

 with insignificant modifications in different arthropods, for 

 instance in scorpions (Pavlovsky, 1917 ; Pavlovsky and 

 Zarin. 1918), in Arachnida (Schimkevitsch, 1884), and insects 

 (lice (Pavlovsky, 1906 ; Sikora, 1916), bugs (Voronkov, 1907), 



