94 (dr. a. C. OtJDEMANs). KOTES ON ACARI. 



tracheae or not, without, however, weighing this character as to 

 the relative ancienty of the two smaller groups. 



My considerations about the Classification of the Acari have 

 already been laid down in the Tijdschrift voor Entomologie, vol. 45, 

 Verslagen, p. 55 — 64. The following is a mere translation of these 

 pages. 



In classifying we have to direct our attention on several facts. 

 Acari with tracheae are certainly older than those without repiration- 

 organs. Acari with a heart are older than those without circulatory 

 organs. Free living Acari with quick motions and predatory qualities 

 are older than free living slow vegetarians; they are also older 

 than Acari which live parasitic on animals; and free living vege- 

 tarians are older than Acari which live parasitic on plants. Acari 

 with chelate mandibles are older than those with claw-shaped or 

 even stylet-like mandibles. And so on. Further we must pay attention 

 to the relation of the smaller groups and unite them to higher 

 groups. 



Undoubtedly the Parasitidae (Gamasidae), Ixodidae and Spelaeo- 

 rhyuchidae are related. They form the group of Mesostigmata; their 

 stigmata are situated behind the 4th pair of legs, or they are moved 

 a little more forward, but they remain always behind the 2d pair 

 of legs; a few Parasitidae and Ixodidae have a heart; in one species 

 of Parasitidae {Rhodacarus^ and in the SpelaeorhyncJiidae the vulva 

 lies behind the 4th pair of legs, like in the spiders. According to 

 these primitive characters I consider the Mesostigynata as a primitive 

 group. 



Undoubtedly the Thrombidiidae, Hydrarachnidae, Tarsonemidae and 

 Halacaridae are related. 



Their two stigmata are situated on the dorsal side of the middle 

 of the capitulum. (In the Tarsonemidae the males have no respi- 

 ratory organs). They form together the group of Prostigmata. I 

 consider the Prostigmata younger than the Mesostigmata, because 

 they have no heart; because the pah" of stigmata has moved 

 far forward, even has passed the first pair of legs, and even has 

 got a dorsal situation; because only a few membres of this group 



