98 



CERTAIN PARASITIC INSECTS. 



executes after having first inserted the labium through a sweat- 

 pore. When the hooks have got a firm 

 hold, the first pair of setae (the real 

 mandibles transformed) are protruded ; 

 these are, towards their points, united 

 by a membrane so as to form a closed 

 tube, from which, again, is inserted the 

 second pair of setae, or maxillae, which 

 in the same manner are transformed 

 into a. tube ending in four small lobes 

 placed crosswise. It follows that when 

 the whole instrument is exserted, we 

 perceive a long membranous flexible 

 tube hanging down from the labium, and 

 along the walls of this tube the setiform 

 mandibles and maxillae in the shape of 

 long narrow bands of chitine. In this way 

 the tube of suction can be made longer 

 or shorter as required, and easily ad 

 justed to the thickness of the skin in 

 the particular place where the animal is 

 sucking, whereby access to the capil 

 lary system is secured at any part of 

 the body. It is apparent, from the 

 whole structure of the instrument, that 

 it is by no means calculated on being 

 used as a sting, but is rather to be com 

 pared to a delicate elastic probe, in the 

 use of which the terminal lobes proba 

 bly serve as feelers. As soon as the 

 capillary system is reached, the blood 

 will at once ascend into the narrow 

 tube, after which the current is contin 

 ued with increasing rapidity by means 

 of the pulsation of the pumping ventri 

 cle and the powerful peristaltic move- 



*lll. Mouth of the Louse, ment of the digestive tube.&quot; 

 If we compare the form of the louse (Fig. 112, Pediculus 



* Figure 111 represents the parts of the mouth in a large specimen of Pediculus 

 vestimcnti, entirely protruding, and seen from above, magnified one hundred and 

 sixty times; aa, the summit of the head with four bristles on each side; 66, the 



