108 On the Physiology o/'Hsematopinus tenuirostris. 



upon the tracheae, by which these are themselves compressed 

 and squeeze out the air contained in them. Expiration is 

 therefore of an active nature. When the air is driven out 

 through the stigmata, the contraction of the respiratory 

 muscles ceases, the body-cavity dilates again, the pressure of 

 the organs upon the tracheae ceases, the latter expand again 

 by the agency of the spiral thread, and fresh air flows in 

 through the stigmata. Landois thinks that the inspiration i3 

 entirely passive. Up to this point certainly ; but now the air 

 that has penetrated into the tracheae has to be driven into the 

 finest capillaries. For this purpose another narrowing of the 

 tracheae is necessary ; but this can be effected only by a constric- 

 tion of the body, and therefore by the contraction of the re- 

 spiratory muscles. In inspiration the above-mentioned closing 

 apparatus of the tracheae is of great importance as Krancher has 

 ascertained. He says (28, p. 516) : — " If this were wanting the 

 animal could not breathe at all, and would consequently be 

 incapable of living. Without the closure of the tracheae the 

 animal, if it wished to breathe, would always expel again the air 

 which is drawn into the body by the corresponding opposite 

 movement, it would never penetrate into the finest ramifica- 

 tions. But when the tracheal closing apparatus comes into 

 action and the tracheae are full of air, the contraction of the 

 body and the narrowing of the tracheae consequent upon it 

 presses the air more or less into their finest extremities, where 

 the exchange of gas may take place in the most favourable 

 manner.'* From what has been said we see that in the Lice 

 both inspiration and expiration are of an active nature. 



EXPLANATION OF PLATE HI. 



Fig. 1. Adult female of Hcematopimis tenuirostris, Burm., seen from the 



ventral surface, showing the tracheal system, the ganglia, and 



the hairs. 

 Fig. 2. Head of Hcematopimis eurystemus, Nitzsch. 

 Fig. 3. Abdominal stigma of H. tenuirostris. 

 Fig. 4. Head and thorax, seen from the side, showing the muscles and 



the cerebral ganglion (d). 

 Fig. 5. Upper extremity of a single muscle, greatly magnified. 

 Fig. 6. Intermediate leg, with the muscles and chitinous thickenings, seen 



from the side. 

 Fig. 7. Antenna, showing the chitinous plates, the thickenings and the 



hairs, and the terminal surface with tactile bacilli. 

 Fig. 8. Head, with the buccal organs (a, b), oesophagus (ee), and eyes (/)- 

 Fig. 9. Buccal organs and muscles, more highly magnified. 

 Fig. 10. Attached egg, with embryo (e) and vitellus (rf). 

 Fig. 11. Upper chamber of an ovarian tube. 

 Fig. 12. Ileum, rectum, and colon, with their membranes. 

 Fig. 13. Upper part of the dorsal vessel, a, tube ; c, appendages ; b, 



efferent duct^ from the tube into the appendages ; d, appendage 



connected with the ovaries. 







