80 



Polyplax miacaniha Speiser. 



This differs from P. spinulosus in having a longer and narrower 

 anterior part of head, in that the last joint of the antennae is more 

 slender, and the antennae are only two-thirds as long as the head. 

 The abdominal segments 4 to 7 show an acute process at the sides. 

 Length, 1.5 to 1.75 millimeters. 



Taken from rats in Abyssinia. 



MITES— ACARINA. 



The mites (order Acarina, class Arachnida) are readily known 

 from the insects (fleas and lice) by having four pairs of legs, no an- 

 tennae, and the abdomen does not show any segmentation, nor is there 

 usually any distinction between head and thorax. In some groups 

 there is a small head-like part, called the capitulum. The mouth 

 parts consist of a pair of mandibles (often styliform or needle-like), a 

 lip, and a pair of palpi. In some forms there is a central piece, called 

 the hypopharynx, and in other' groups is a plate above the mouth 

 parts, known as the epistome. The body usually shows more or less 

 distinctly a division into two parts — the anterior, called the cephalo- 

 thorax, and the posterior the abdomen. However, in many mites it 

 is not possible to separate these parts, except that it is considered 

 that the legs are borne by the cephalothorax. In many forms 

 there is a small, simple eye each side on the cephalothorax, but 

 many other forms are blind. Some species have a tracheal system, 

 which opens in a pair of spiracles near the hind legs or near the 

 anterior end of the body; other species have no definite respiratory 

 system. The genital aperture is on the venter, usually between the 

 legs. The legs consist of the usual joints — coxa, trochanter, femur, 

 tibia, sometimes a metatarsus, and a tarsus. The tarsus terminates 

 in a pair of claws, sometimes three or only one, and often a sucker or 

 caroncle. Most mites are not parasitic; those species that are para- 

 sitic are often free in one stage. The parasitic mites suck the blood 

 of their host, feed on the hair or dermal scales, or burrow in the skin. 

 Some predaceous species inhabit animals to hunt and eat the parasitic 

 mites that infest that animal. 



The mites that occur parasitically on rats belong to four families: 

 Sarcoptidae, Cheyletidae Ixodidae, and Gamasidae. 



1 . A distinct spiracle or breathing pore on each side of body near coxee III or IV 2 



No such spiracle or pore visible 3 



2. A small, distinct head part in front of the body; palpi three jointed; a granulate area 



around the spiracle ; no sternal plate Ixodidse. 



No such head part; palpi five jointed; no granulate space around spiracle, but a 

 long, chitinized piece reaching forward from it; a more or less distinct sternal 

 plate Oamasidse. 



3 . All legs simple, unmodified, ending in a stalked sucker Sarcoptidse, 



Front legs short, enlarged, and modified for clasping; all legs end in one'or two stoiit 



claws Cheyletiix. 



