50 BRITISH TYROGLYPHID.fi. 



genus Sarcoptes and its allies (the itch-mites), and the 

 greater part of the Hydrachnidae ; with regard to these 

 Acari the definition is perfectly correct ; a similar, or 

 almost similar, condition is found amongst the Tyro- 

 glyphidge in the genera Chortoglyphus, TrichotarsuH, 

 Hericia and Carpoglyphus, and many, but not all, 

 members of the genus Glycyphagus ; but in almost 

 the whole of the remainder of the Tyroglyphidas, the 

 whole of the Oribatidge, and by far the greater number 

 of the Gamasidae the Trombididse and the Phytoptidae, 

 constituting much the larger portion of the Acarina, 

 there is a sharp constriction of the body between the 

 second pair of legs and the third if present ; the portion 

 of the creature anterior to this constriction is called the 

 cephalothorax by acarologists, and the portion posterior 

 to it is called the abdomen. Considerable difference of 

 opinion has existed, and indeed still exists, as to the 

 propriety of calling this portion of the body the abdo- 

 men. Some comparative anatomists consider that in 

 the Tracheata nothing should be called the abdomen 

 which is not legless, and they quote the instance of the 

 spiders in the Arachnida; undoubtedly in the great 

 bulk of the Acarina, what are known by the name of 

 the two posterior pairs of legs are behind the constric- 

 tion. This does not appear to me to be decisive 

 because in other great groups of Arthropoda the abdo- 

 men does bear appendages which are used for walking 

 or swimming, and I do not see any reason why it 

 should not do so in the Acarina ; although it might raise 

 a question whether the third and fourth pairs of legs in 

 the Acarina can be considered homologous with those of 

 the spiders, etc. In the Phytoptidae and Demodicidaa 

 the abdomen is legless, and is vermiform in character ; 

 but in the former of those families only two pairs of 

 legs are present, the two hind pairs being probably 

 lost, either from the mode of life or otherwise. Again 

 it is said that the constriction is merely external and 

 does not indicate the arrangement of the internal 

 organs into two entirely separate groups ; but it seems 



