August. 1913. 



KNOWLEDGE. 



311 



The reader need scarcely be reminded of the importance c' 

 the proper choice of the aperture of the condenser, and to 

 realise how different are the effects produced by the different 

 apertures one need only instance the case of Koch's 

 structural and colour images : both are the result of the same 

 optical condition — in one case with a contracted diaphragm to 

 attempt to bring out fine structural details in unstained 

 preparations, in the other with the condenser opened to its 

 full extent to show deeply stained bacteria. 



A condenser whose aperture can be varied in a numerically 

 definite manner does away with loose statements to the effect 

 that this or that investigation should be carried out with the 

 condenser shut down to one third, a half, two thirds, and so 

 on. Directions of this kind are far too indefinite, and are not 

 even applicable to condensers of different maximum apertures. 

 In future it will be as easy as it will be desirable to specify the 

 aperture of the condenser among the other particulars relating 

 to microscopic observations, projection, and photomicrography, 

 such as the denomination of the lens, its magnification, and 



the tarsi has two claws with a caruncle or sucker ; this 

 caruncle assumes different forms in different species. 



They prefer damp places to live in ; that is to say, those 

 that are not parasitic. They are found under stones, decayed 

 wood, garden rubbish, stables, under loose bark of trees, 

 in moss, and I once found a large number in a deserted robin's 

 nest. 



The Gamasoidea is divided into three families — Gamasidae, 

 Uropodidae and Dermanyssidae. Nathan Banks, the 

 American writer on mites, gives the following key to the three 

 families of the Gamasoidea : — - 



(1) Parasitic on vertebrates : mandibles fitted for piercing ; 



body sometimes constricted. Dermanyssidae. 



Free, or attached to insects, rarely on vertebrates, never 



on birds. 2. 



(2) First pair of legs inserted within the same body-open- 



ing as the oral tube : genital apertures surrounded 

 by the sternum. Uropodidae. 



Figure 340. 



Gamasus equestris Koch. 



Drawn under camera lucida from a 

 specimen found at Barmouth. $ 



Figure 341. 



Uropodidae. 



Drawn from a specimen found in moss 



at Sunningdale. Ventral surface of 



Cilliba cassidens. ? 



Figure 342. 



Dermanyssus avium. 



Drawn under camera lucida from 

 specimen from a canary. 



the nature of the illumination, time of exposure, and so on. 

 This will be an easy matter, as the aperture can be read by 

 the index of the iris-diaphragm. 



C. Metz. (Wetzler.) 



GAMASOI DEA. — Anotherneglected super-family of mites is 

 the Gamasoidea, and yet entomologists are continually coining 

 across specimens, either as messmates or parasites, on various 

 kinds of insects. Not much notice, however, is taken of thein. 

 They are more often than not thrown away as a nuisance ; yet 

 they may be more interesting than their host. The most 

 commonly known of all the Gamasids is no doubt Gamasus 

 coleoptratorum and G. crassipes, so often found in large 

 numbers on the Dor beetle and others. Beetles, however, are 

 not the only insects on which they are found ; all other sorts of 

 insects are pressed into their service either to supply them 

 with nourishment or as a means of getting from one place to 

 another. One family of the Gamasoidea — the Dermanyssidae 

 — are all parasitic on warm-blooded animals. 



The characteristics of the super-family are — they are without 

 any visible eyes ; they are mostly pale coloured : being 

 generally a light brown or fawn colour they have none of the 

 brilliant reds we find in the Trombidiuins. The bodies are 

 broad and flat. The skin is mostly smooth and tough, in 

 some species chitinous only in parts, the other parts being 

 thinner and paler in colour. The legs have seven segments, 



(3) First pair of legs inserted at one side of the mouth 

 opening, male genital aperture usually on the anterior 

 margin of sternal plate. Gamasidae. 



The Dermanyssidae are all parasitic. The best-known 

 representative is the red mite, found in such large numbers in 

 bird-cages and fowl-houses that have not been kept clean. 

 This bright red colour of the mite (which is commonly known 

 as Dermanyssus avium Dug.) is due to the amount of blood 

 which the mite has sucked from its host during the night. In 

 the daytime, if it can possibly hide away, it is invisible. The 

 Dermanyssidae is split up into about half a dozen genera. 

 There are several papers on this family, but I have not heard 

 of any monograph. 



The Uropodidae. — The most striking feature about this 

 family is that some of its members are found attached to 

 their host with a connecting filament which De Geer 

 thought was of the nature of an umbilical cord by 

 which the mite drew its nourishment. Other naturalists 

 thought it was a silken cord with which the Uropoda tied 

 itself to its host to prevent it being brushed off. It has 

 since been found to be connected with the anus of the 

 mite, and to be nothing more or less than its consolidated 

 excrement : this connection the mite can sever at will. 

 Banks says that these mites so attached are not true 

 parasites, but that it is a means used by nymphs as u 



