108 BRITISH TYBOGLYPHIMI. 



sembles that of G. platygaster, but without the rounded 

 swelling, and the sub-oesophageal ganglion is much the 

 longer, although the supra-oesophageal is considerably 

 the thicker. In Tyroglyphus lonyior Nalepa* draws the 

 brain somewhat of the same shape as in G. platygaster ^ 

 but without the rounded swelling, and with the sub- 

 oesophageal ganglion much the longer of the two, and 

 somewhat concave on its dorsal surface : in reading 

 the expressions dorsal and ventral in connection with 

 the brain, it must be remembered that in the Tyro- 

 glyphidae the two ganglia do not lie in a perpendicular, 

 but in a diagonal line ; the sub-oesophageal being below 

 and behind the supra-oesophageal, and extending almost 

 horizontally backward. If seen from below, or in hori- 

 zontal section through the sub-oesophageal ganglion, 

 the brain of Hericia Bobini shows an irregularly octa- 

 gonal form, the boundary-lines of the octagon being 

 concave, not straight (PL A, fig. 8). It is to be ob- 

 served that, in some species at all events, the form of 

 the brain is not identical in the two sexes ; this may 

 be noticed in transverse sections of that of G. platy* 

 (jaster (PI. A, figs. 11, 12). 



The brain in all Acari which I am acquainted with 

 has an external layer of small, round, cortical cells 

 which stain very deeply ; and an inner, more fibrous 

 mass ; which does not stain nearly so readily. In the 

 TyroglyphidaB that I am acquainted with the cortical 

 layer is very thin, usually only one cell thick. 



At the end of my paper on the " Brain in Acarina " 

 above quoted will be found a table giving the approxi- 

 mate proportionate volume of the brain as compared 

 to that of the whole creature in selected species from 

 various families of Acarina; the proportion which the 

 bulk of brain bore to that of the whole creature in 

 OlycijphaguB platygaste v , which was the species selected 

 from the Tyrogiyphidse, was *19 per cent. ; this is far 

 the lowest of any family that was tested ; the highest 

 proportion was 1*61 per cent, in Gamasuv teri'ibUis, and 



* PL A, fig. 3. 



