THE acari constitute a large order of minute animals, including mites, ticks, itch insects, etc, with which, in some of their forms, every one is more or less familiar, though, owing to their small size and obscure ways of living, but little is known of their structure and habits. They are often spoken of as insects, but are by scientific classification separated from the true insects, the most marked distinction being the possession of eight legs instead of six. Usually they are furnished with a suctorial apparatus. They are parasitic on both the animal and vegetable kingdoms. No class of animals is free from them. They prey upon each other; insects are infested with and support them at the cost of their lives; they attach themselves to fish and cold-blooded reptiles, and colonize on the whole range of birds and mammals. Some live under the skin, burrowing in the muscles; others dig corridors beneath the epidermis; while others, again, wander only on the surface. They are found in the lungs, air-passages, and intestinal canals of vertebrates. They are very prolific, and, though small, awkward, and slow-moving, they are transferred in a surprising way, often suddenly appearing about houses in enormous numbers, infesting plants and domesticated animals, or swarming on articles of food. Sometimes, especially in the tropics, they inflict considerable suffering; but it is probable that, on the whole, they are beneficial to man. A few directly injure him, and none directly benefit him, but indirectly they do him service by preying, as will be seen, on insect-pests and by acting as scavengers. The number of species in this family is so great as to forbid any detailed description, or even the merest mention of all of them. For the most part they are seldom seen except by those who make them a study. But a brief account of what is known about those of them that come into relations with man closely enough to affect his comfort and welfare, may be of interest and value.
The species thus referred to are found in the following classes: 1. Spinning and harvest mites, red spiders, etc. (Trombidiinæ); 2. Insect mite-parasites, sometimes called ticks (Gamasidæ); 3. True ticks (Ixodidæ); 4. Cheese and sugar mites, itch mites, etc. (Acaridæ)—the portion of the family commonly included under the name acari.
The spinning mites, like their relatives the spiders, can spin webs. They are of small size and semi-transparent. The most common is the red mite (Fig. 1), Tetranychus telarius, so well known to all who have the care of house plants. It is one twentieth of an inch long, yellowish, with two red spots on the sides, though its color