20 
Transactions of the Society. 
founded the genus Acarus, and to the present state of our knowledge 
on the subject ; not going more than I can avoid into the dry details of 
classification, but rather illustrating it by such of the more striking 
examples and incidents in life-histories as I may hope that you may be 
able to listen to without weariness. The genus Acarus was, as I 
said, originated by Linnaeus, but the word was not ; it is far older. 
It is found in Aristotle in its Greek form, and was probably originated 
by him ; it is derived from arcapys, uncuttahle. The name was not 
by any means a bad idea on Aristotle's part ; the wonder is, not that 
he thought them uncuttahle, hut that he ever saw them at all ; but 
naturally, in spite of his marvellous ability, such things as Cambridge 
rocking microtomes were not dreamt of in Aristotle’s philosophy, and 
I now hold in my hand a set of over 120 serial sections cut from a 
far smaller “ uncuttahle ” than Aristotle can ever have seen. 
When one has a word the next thing one desires to know is what 
it means, and therefore a clear definition of what an Acarus is is 
requisite, but it is far from easy to obtain. If we omit those few 
puzzling forms of life which occur on the borderland of almost every 
group, probably no acarologist when he looks at a living creature has 
the least difficulty in saying whether it is an Acarus or not ; but if 
he comes to give his reasons and formulate an accurately worded 
definition it is a very different matter. If we refer to any of the text- 
books of zoology we shall find one leading idea. Claus (English 
translation) defines the Acarina as “ Arachnida with stout body, the 
abdomen unsegmented and fused with the thorax, &c. Lang, one 
of the latest, as “ Abdomen fused with cephalothorax, body unseg- 
mented.” In every one the want of segmentation, and especially the 
fusing of the abdomen with the cephalothorax, is the important part 
of the description ; this sounds very clear, but unfortunately, when 
one becomes well acquainted with the group, it dawns upon one that 
the unsegmented condition must be taken in a limited sense. It is 
undoubtedly true of the greater number of species ; but it has of late 
years been pointed out by Kramer and others that certain Acari, par- 
ticularly when in an immature condition, do show very decided signs 
of segmentation of the abdomen. It may be, and is, said with perfect 
truth, that the mere transverse ringing'of the cuticle is not a proof of 
segmentation, but when each ring has its own set of muscles it 
becomes suspiciously like it. The other and principal distinction, the 
fusing of the cephalothorax and abdomen, is perfectly correct if such 
creatures as some of the freshwater Mites (Hydrachnidae) or the Itch 
Mites (Sarcoptidae) be the examples taken ; but if, on the contrary, we 
look at the great families of the Tyroglvphidae (Cheese- Mites), the 
Trombidiidae, the Oribatidae, the Phytoptidae, and many others, the 
distinction between cephalothorax and abdomen appears sufficiently 
plain. Take such a species as Tyroglyphus corticalis , described by 
me in the Journal of this Society; it hardly seems possible to deny 
that there is a plain distinction between cephalothorax and abdomen, 
