650 E. N. PAVLOVSKY 



tracheae. These latter resist the pressure owing to the elasticity 

 of the included air (which acts only when the breathing 

 apertures are closed), as well as in consequence of the structure 

 of the tracheal walls themselves. The taenidia having the 

 structure of pipes resist any mechanical influence, in particular 

 pressure, more completely than would taenidia of the same 

 diameter if they were solid. An analogous principle in the 

 resistance of materials is applied in the engineering practice. 

 The forms of taenidia are not the same in different insects. 

 Passing over the details we might point out that the taenidia 

 of hemipterous Z a i t h a f 1 u m i n e a have the form of a groove, 

 which structure gives the trachea a mechanical advantage. 



On the Integument and its Appendages. 



The body of the larva of H. caraboides is thickly set 

 with hairs. These are of several kinds : 



(1) Fine hairs ; the most numerous on the segments and the 

 pleural appendages of the abdomen. 



(2) The long terminal threads of the pleural appendages. 



(3) The scarce setae. 



(4) The setae on a pigmented and elevated base. 



The latter are arranged in fours on the tergites of each 

 abdominal segment, forming together four longitudinal rows. 



In connexion with its dense hairiness the integument of the 

 larva of H. caraboides possesses a high degree of sensibility. 

 It is sufficient to touch one of the long hairs of the pleural 

 appendages of the abdomen (Text-fig. 1) to make the larva 

 instantaneously turn its head to the side where the irritation 

 comes from and to seize the disturbing object with its mandibles. 



These organs are useful in two respects : (1) They serve the 

 larva for self-defence, as its whole body with the exception of 

 the head and perhaps also of the thorax is very soft, easily 

 vulnerable, and accessible to the attacks of different carnivo- 

 rous inhabitants of fresh water ; receiving due notice of any 

 possible danger, the larva gets time to put its defensive organs 

 into action, i. e. its sharp and strong mandibles. (2) If a weaker 

 or harmless creature happens by chance to touch one of the 



