200 MR. A. D. MICHAEL ON AjS [Mar. 5, 



pharynx, but he does mention a median nerve which he says goes 

 to a central unpaired eye which he seems to have found in Hydro- 

 droma. Croneberg says that the pharynx in Eylais is innervated 

 from the first pair of nerves from the supra-cesophageal ganglion, 

 which also supply the mandibles : this certainly is not the case in 

 Thyas petrojMhis \ although, as will be seen below, I think that 

 the mandibles are supplied much as Croneberg says. 



It is very difficult to trace the finer nerves in the Acarina and 

 to be certain that one has traced all that start from the brain, 

 although the larger ones, such as the great nerves to the legs, are 

 easily followed ; but to the best of my judgment I have been able 

 to trace, in addition to the azygous nerves, 4 pairs which arise from 

 the supra-cesophageal portion of the mass, 1 pair which arise 

 exactly on the level of the oesophagus but considerably to the 

 side of it, so that I cannot say whether they are supra- or sub- 

 cesophageal, and 5 pairs of large nerves, from the sub-cesophageal 

 portion of the mass. 



The first pair from the upper ganglion are a thin pair of nerves 

 (fig. 20, nm.) near to the median line, and they appear to me, in 

 the present species at all events, to supply the mandibles only — 

 not the mandible and pharynx, as Croneberg says they do in 

 Eylais. Schaub states that the mandibles in Hydrodroma are 

 innervated by the same nerves as the palpi ; this does not seem to 

 me to be the case in Thyas pctrophilus. As regards homologies in 

 other families of Acarina, Winkler, in the Gamasidse, where the 

 mandibular nerves are conspicuous, found that the mandibles were 

 innervated by special nerves not identical with those serving the 

 palpi, and fairly corresponding with the pair I find in Thyas, 

 although situated a little further back, which may probably be 

 accounted for by the great retractility of the mandibles in Gamasus. 

 Nalepa found the mandibles of the Tyroglyphidae to be innervated 

 by special nerves, different from those serving the palpi, and 

 agreeing in position with those I am now describing. Henkin 

 also apparently found the same thing in Tromhidium fuliginosum. 



The second pair of nerves from the supraroesophageal portion 

 of the brain arise somewhat from the dorsal suface of that organ; 

 they are an extremely thin pair (fig. 20, vv.) and innervate the 

 muscles which run from the dorso-vertex ' to the maxillary lip and 

 possibly other dorso-ventral muscles. 



The third pair of supra-cesophageal nerves spring from nearer 

 to the anterior edge of that region of the brain and are the large 

 optic nerves (fig. 20, no.). These have been well described and 

 figured by Schaub ; they are long and large nerves, each dividing 

 dichotomously near the distal end, and sending one branch to each 

 of the two eyes on that side of the body, which are pressed so 

 closely against each other as to appear like one double eye. 



The only difference of any importance which I have found 



^ The dorsal exoskeletou of the posterior part of the cephalothorftx. 



