514 Observations on Habits and Parasites of Common Flies 
occurred in empty pupa cases. According to Michael (1901) the 
Gamasidae feed on the Tyroglyphidae, and it seems possible that some 
mites attach themselves to flies in order to feed on the hypopial nymphs, 
which so often adhere to the latter. 
Pseudo-scorpions. 
Occasionally small lobster- or scorpion-like creatures, about 2-5 mm. 
in length, are found clinging to the legs of flies. Though the records 
of their occurrence on flies go back for more than 150 years, it is, except 
in certain seasons, an uncommon event to find one attached to a fly. 
Judging from the published records, the house-fly is the species on which 
they are most frequently noticed. During the latter part of the summer 
of 1915 the writer was fortunate enough to find these creatures attached 
to different species of flies, and to make some observations on them. 
It may therefore be a fitting opportunity to give a short account of 
their structure and habits, and of the views which have been put forward 
to account for the very curious habit possessed by certain species of 
attaching themselves to flies. 
These creatures are arachnids, belonging to the order Chernetidea 
(pseudo-scorpions). The body consists of a cephalothorax without 
segmentation in some groups, but with one or two dorsal striae in 
others, and an abdomen composed of twelve segments. The segmenta¬ 
tion of the abdomen is emphasised by the presence of chitinous plates 
dorsally and ventrally, but the last two dorsal plates are fused, so that 
ordinarily only eleven segments can be counted dorsally and nine 
ventrally. The eyes, when present, are either two or four in number, 
and are placed near the lateral borders of the carapace towards its 
anterior end. They are whitish, and only very slightly convex, and 
are never situated on prominences (Warburton, 1909). Some species 
are without eyes. They possess pedipalpi, chelicerae and four pairs of 
legs. The chelicerae, which are two-jointed and small, bear certain 
interesting but minute structures, the galea, serrula and flagellum, used 
by some authorities on the group for the purposes of classification. 
The very large, six-jointed pedipalpi, terminating in claws, give 
these animals their superficial resemblance to scorpions or lobsters. 
These structures have gained for them the popular titles of pseudo¬ 
scorpions or chelifers. About twenty species occur in Britain, some 
of which are stated to be common and widely distributed, but are 
seldom seen unless specially looked for in their haunts under stones, 
beneath the bark of trees or among moss, debris, decaying substances 
