TRÄGÅRDH, MORPHOLOGY AND PHYLOGENY OF PARAS ITID.E. 13 



Greeniella lives on a bee, Xylocopa, Liponyssns, Spintur- 

 nix and Ptilonyssiis on different mammals; hence it seems 

 likely that in their c a se the reduction in length, or entire 

 absence, is due to their parasitic mode of life. Which rela- 

 tion tills feature has to the parasitic life, is a question im- 

 possible to answer, since no investigations ha ve as yet been 

 made into the function of the organ; whether it has a respira- 

 toric or sensorial function, or both. 



Nordenskiöld has, however, proved that in the peri- 

 trema of the Ixodince, there are, below the cuticle, numerous 

 sensorial cells; and if the peritremata are chiefly sensorial 

 organs, their retrogressive development is easily understood 

 as regards the parasitic forms. 



In the case of Epicrius, Iphis and Zercon, on the other 

 hand, which are all free-living, it is seems difficult to explain 

 the absence or shortness of the peritremata as anything else 

 than a primitive feature. 



This theory is, as in the case of the segmentation of 

 the dorsal side, supported by facts gathered from the onto- 

 geny of those species which are known in this respect. 



Thus OuDEMANS has pointed out, that in the proto- 

 nymphae of the gemis Parasitus the peritremata are verv 

 short, but grow in length successively after each moult; and 

 the same applies also very likely to the other genera with 

 long peritremata, although nothing is yet known about their 

 postembryonic development. 



On the ventral shields. 

 a. Position of the liairs. 



In order to understand the different types, as regards 

 the relation of the different shields, which we are able to 

 discern amongst the Parasitidce, it is imperative to notice 

 the position of the hairs. This may sound as an exaggera- 

 tion, but is nevertheless true. The merit of discovering the 

 value of the hairs for the comparative morphology belongs 

 to OuDEMANS, who always takes this feature into considera- 

 tion in his papers. whereas the older acaridoiogists, and also 



