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Show 200 MB. A. D. MICHAEL ON AN [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 m Hydrodroma. Croneberg says that the pharynx in Eylais is innervated from the first pair of nerves from the supra-oesophageal ganglion, which also supply the mandibles : this certainly is not the case in Thyas petrophilus; 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 m y judgment I have been able to trace, in addition to the azygous nerves, 4 pairs which arise from the supra-cesophageal portion of tbe 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-oesophageal, 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 petrophilus. A s regards homologies in other families of Acarina, Winkler, in the Gamasidae, 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 a m now describing. Henkin also apparently found the same thing in Trombidium fuliginosum. The second pair of nerves from the supra-cesophageal portion of the brain arise somewhat from the dorsal suface of that organ; they are an extremely thin pair (fig. 20, nu.) and innervate the muscles which run from the dorso-vertex a 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 exoskeleton of the posterior part of the cephalothorax. |
