TRACHEATA 421 
as the mange or the itch. The female Swrcoptes scabei makes a 
tunnel in the skin and sits at the inner end, pushing the eggs 
towards the exterior as they are 
laid. 
The PHYTOPTIDAE are gall mites, 
and make amongst others the curious 
conical excrescences which are so 
common on the leaves of lime trees, 
maples, etc. In many of them the 
posterior pair of legs are replaced by 
bristles. 
The HyYDRACHNIDAE or water- 
mites are usually brightly coloured. 
They often hang on to water-insects, 
and may be frequently found attached 
by their rostrum to Dytiscus, Nepa, 
etc. One species, Atax bonzi, is 
common in the mantle-cavity of Unio. 
Fic. 242.—Sarcoptes scabei (the 
Itch Mite), female. After 
Meguin. 
The TROMBIDIIDAE are brightly-coloured mites, whose larvae 
Fig. 243.—Izodes aegyptius, Savigny. 
a. Chelicerae. d. Thorax. 
6, Pedipalpi. jf. Abdomen. 
c. Head. 
often live parasitically on spiders or insects. Some of them, 
as Tetrarhyncus teleariuvs, commonly known as the red spider 
