The President's Address. By A. D. Michael . 
23 
tion the single Linnaean genus of a few species has expanded into. 
Professor Canestrini of the University of Padua, a well-known and 
able zoologist, and a specialist in Acarology, in his classification which 
was the last before Trouessart’s, contends that the differences between 
the Acari and the other Arachnida are too great to allow both to he 
included in one class, and he proposes raising them to a separate 
class, which he divides into six orders and thirty-five families. He 
was not, however, the first to regard them as a separate class, Haller 
and Oudemans had previously suggested the same thing. The prin- 
cipal reasons which impressed these respective writers were that 
Acari in the course of their life-histories mostly undergo transforma- 
tions which have not any parallel in other Arachnida ; that many of 
them pass through what Claparede called the “ Deutovum ” stage, 
wherein the hard external chorion of the egg splits longitudinally 
into ,two boat-shaped halves, which separate, allowing the lining 
membrane to become the exterior casing, and thus considerable increase 
in the size of the maturing egg is permitted ; that some of their 
life-histories include the curious Hypopal stage unknown in other 
creatures ; and certain embryological reasons ; besides, possibly, the 
difficulties above alluded to. Trouessart felt, I think rightly, that 
the Acari were too closely allied to other Arachnida to be excluded 
from the class ; but yet he thought that the difference between them 
and the other orders of that class was greater than that of those 
orders from each other, and therefore he treats them as a sub-class ; 
he makes 10 families, 24 sub-families, and 212 genera ; this is what 
Linnaeus’s single genus has come to ; yet Trouessart rejects numerous 
genera of other authors, and new genera are springing up every 
month, although the Acari of the greater part of the world, with the 
exception of the Ticks, are still almost entirely unknown. 
Before referring to any points of interest confined to particular 
families, I will mention what seem to me to be one or two affecting 
the whole order (or sub-class in Trouessart’s view). A picture was 
lately shown at one of our exhibitions entitled “ Eyes and No Eyes ; ” 
now this just expresses one of my supposed interesting points. If we 
take an Erythrseus or a Bdella we shall find it provided with at 
least one pair of well-developed and conspicuous eyes. Both are very 
active predatory creatures, and you will probably say that the posses- 
sion of eyes is what would be expected. How could they possibly 
catch their prey without ? But most Gamasidae and many Cheyletidae 
are equally active and equally predatory, but they are entirely devoid 
of anything which we can recognize as an organ of vision ; yet they 
catch such agile things as Thysanuridae in the open, without con- 
structing any web or trap. How do they do it ? That appears a 
mystery. A rather good and amusing example of this difficulty 
recently came under my notice. I have of late years paid a good 
deal of attention to the Gamasidae, chiefly in respect to their habits 
and anatomy. I collected one or two Holotaspis belonging to a 
