566 MEDICAL ENTOMOLOGY 



therefore only seen in the female ; in the Argatini it |is situated 

 in the anterior end of the body above the capitulum and beneath the 



anterior diverticulum (Ornithodorus), and consists of a 

 Gene's Organ 1M . , T ...... 



single sac-like gland ; in the Ixodini it lies below the 



anterior end of the scutum and usually consists of four separate tubes. 

 The gland follicles, which are lined by a single layer of columnar cells, 

 'only attain their full development just before the female begins to 

 oviposit. In the Argatini the gland opens externally by a large trans- 

 verse slit in the fold of skin dorsal to the capitulum ; in the Ixodini 

 the glands open by separate ducts in the fold between the capitulum 

 and the scutum ; their openings can be demonstrated by folding the 

 capitulum back on to the ventral surface. The function of the sticky 

 secretion which is discharged from Gene's organ is dealt with in con- 

 nection with the process of oviposition. 



/The respiratory system, unlike that of the Insecta, consists of a single 



orifice on each side, situated on the stigmal plate, with a number of 



tracheae for the supply of the organs. In the Argatini 



The Respiratory the stigmal plate is usually small, and is situated a little 



8jf8 |ate' Plate^LXXv' anterior to coxa IV ; in the Ixodini t is large and is 



figs. 4 and 5 situated behind and below coxa IV. The structure of 



the stigmal plate of HaemaphysaUs pnnctata has been 



described by Nuttall in collaboration with Cooper and Robinson ; that of 



Ixodes ricimis by Samson ; their observations are here summarized. 



The spiracle consists of an elevated chitinous plate of varying shape 

 land dimensions and with a well-defined margin ; in the adult stage it is 

 longer than broad, while in the nymph the reverse is the case ; the larva 

 has no spiracles. The shape of the chitinous plate is of use in separat- 

 ing the genera, and its structure in recognizing the species, at least of 

 the genus Dermacentor. Situated a little to one side of its centre and 

 nearer the antero-mesial margin there is a dark raised area, named by 

 Nuttall, Robinson and Cooper the macula ; it is elliptical in outline 

 and is pierced in the female by a slit-like opening termed by the above 

 observers the ostium (stigmal pore of other authors), which is guarded 

 externally by a raised lip acting like a valve ; the ostium is absent in the 

 male and the nymph. The marginal portion of the spiracle is of a 

 dark colour, while the area between it and the macula is of a greyish 

 colour and has many evenly distributed p6res opening on its surface. 

 On examining the surface of the spiracle with a lens dark and light spots 

 may be made out ; the latter represent the small pores of the superficial 

 layer to be .described presently, and the former the rods or pedicels 



