56 BRITISH TYROGLYPHID.E. 



the distal joint however is often truncated, sometimes 

 even enlarged at the end ; it often bears a sensory 

 papilla and a few small hairs, probably tactile. There 

 are seldom above one or two hairs on the other joints, 

 but these may be larger. 



As is the case with the mandible, so also with the 

 palpus. That of Histiostoma differs almost entirely 

 from that of all other genera of the family, although, 

 of course, really modified from the type form ; but the 

 palpus of Histiostoma is as actively functional an organ 

 as that of any Acarus. Megnin considers that there 

 are only two free joints, and that the third, usually 

 found in the family, is wholly anchylosed to the hypo- 

 stome ; it appears to me, however, that there is a third 

 free joint, but small, and of far less importance than is 

 usual in the family. The two distal joints of the 

 palpus bear a thin, broad, and transparent membrane ; 

 sometimes running their whole length, sometimes a 

 part only (PL III, fig. 1, p. m. fig. 20 ; PL IV, fig." 7) ; 

 it may surround both sides of the distal joint. In H. 

 rostro-s erratum this membrane runs along the hypo- 

 stome also, and the membranes from the two palpi meet 

 in the middle of the hypostome and join ; thus forming 

 one continuous, broad, undulating margin ; which turns 

 over somewhat on the hypostome, and forms a funnel 

 leading into the mouth ; or rather a half -funnel ; for it 

 is not closed above ; even in those species where the 

 membrane does not run on to the hypostome it directs 

 the nutritive fluids towards the mouth. On the distal 

 joints of the palpus are two flagella; the anterior is 

 usually the larger where there is a difference ; they 

 are turned almost at right angles ; the anterior is 

 constantly in motion when the creature is feeding, 

 beating regularly and rapidly ; these flagella cause 

 the current which carries the food-material into the 

 mouth. 



The Lingua is an organ not much developed in the 

 Tyroglyphidse ; where present it is a simple, rather 

 narrow, triangular membrane arising from the floor of 



