INTERNAL ANATOMY. 115 



rest of the skin at each ecdysis. This vesicle is very 

 conspicuous in some species where the cuticle of the 

 abdomen is smooth and transparent, such as Rhizo- 

 glypkus echinopus, Aleurobius farwse, Tyroglyphus myco- 

 pliagus, T. Wasmanni, etc. It may be seen in all 

 stages subsequent to the egg, and indeed is often seen 

 in the hypopus when it cannot be seen from the 

 exterior in the living adult, as in Glycyphagus Crameri. 

 The form of the organ is usually hemispherical, or 

 lenticular, but in the adult it generally presents its 

 edge, or nearly its edge, to the observer looking from 

 the dorsal aspect ; it is practically a membranous sack 

 filled with a highly refractive oily liquid. This vesicle 

 communicates with the exterior surface of the abdomen 

 by a short narrow canal which ends in a small round 

 hole or mouth in the cuticle, often surrounded by a 

 chitinized ring or ridge. A precisely similar organ 

 exists in the larvae and nymphs of many Oribatidae, 

 such as Oribata lapidaria, 0. setosa, and 0. orbicularis. 

 It persists in many of the adult Oribatidae, such as Her- 

 mannia convexa (picea), H. bistriata, Nothrus palustris, 

 N. spinifer, and N. sylvestris ; in Hermannia arrecta 

 the mouth is surrounded by a chitinous tubular pro- 

 jection instead of a mere ring, which makes it a con- 

 spicuous object. The organ probably exists in many 

 other Oribatidge where it has not been seen. 



Considerable difference of opinion has arisen from 

 time to time among acarologists as to the function of 

 this organ ; probably Robin was the first who saw and 

 described the vesicle ; * his species was Tyroglyphus 

 siro. He called it a " vesicula pleine de liquide in- 

 colore," but he did not discover its passage to the 

 exterior, nor did he suggest a function. Pagenstecher t 

 saw the external opening of the passage in Aleurobim 

 farinas (which he calls T. siro), but he took it for a 



" Memoire zool. et anatom. sur di verses especes d'Acariens de 

 la Famille des Sarcoptides," in 'Bull. Soc. Moscou,' 1860, t. xxxiii, p, 

 292, pi. viii, fig. 2, i. 



f "Einiges znv Anatomic von Tyroglyphus siro" in ' Z. wiss. Zool.,' 

 1862, Bd. xi, p. 122, Taf. xiii, fig. 3. 



