116 BRITISH TYROGLYPH1D&. 



stigma ; although he says that there are not any tracheae. 

 Turpin apparently saw the vesicle in what probably 

 was Glycyphagus domesticus,* and Fiirstenberg saw 

 it in some Sarcoptidas ; f but these authors took the 

 organ for an air- sack, which is strange considering 

 that it is almost always full of liquid unless the con- 

 tents have been artificially expelled. Furstenberg 

 even describes a regular motion of expansion and 

 contraction in the vesicle, which no one else has ever 

 been able to see. ClaparedeJ considered the organ to 

 be an excretory gland ; and practically to represent the 

 Malpighian vessels of the Gamasidas, Trombidiidas, 

 Ixodidaa, Hydrachnidaa, etc., although not homologous 

 to them, and he says that the two organs never exist 

 in the same species, which he gives as an important 

 argument in favour of his view ; but in this the 

 eminent Swiss naturalist made a mistake, for both 

 exist in the very species he was writing about, viz. 

 Tyroglyphus siro and T. longior ; and probably in most 

 if not all other Tyroglyphidae. The Malpighian vessels 

 are clearly homologous with those of the Gamasidae 

 (see p. 73), but there still is this point in favour of 

 Claparede's argument, viz. that in the Tyroglyphidas 

 the Malpighian vessels are very small, sometimes 

 almost rudimentary, instead of being large and im- 

 portant organs, as they are in the Gamasidas. Nalepa 

 entirely repudiates all previous suggestions about their 

 function, and declares them to be oil-glands which 

 serve to keep the skin lubricated; he suggests that they 

 would thereby, inter alia, prevent the creatures sticking 

 to the various sugary and other adhesive substances 

 upon which they are so often found ; he calls the organs 

 the oil-glands. Claparede had mentioned the contents 

 as thick and oily. In my own work on the Oribatidas, || 



* ' C. R. Ac. Sci.,' 1837, t. v, p. 672. 



t ' Die Kratzmilben von Menschen und Tliiere,' Leipzig, 1861, 

 p. 192. 



J " Studien an Acariden," in ' Z. wiss. Zool.,' 1868, Bd. xviii, pp. 

 498, 499. 



'Abth.i,' pp. 204-206. 



|| 'British Oribatidae,' London, Ray Society, 1884, p. ISO. 



