PART V.--PARASITIC DISEASES



CHAPTER XXIV

PARASITIC INSECTS AND MITES


Parasitic insects are common causes of skin diseases in domestic animals.
The diseased conditions of the skin, and the irritation that they may cause
the animal, depend on the life history and habits of the parasite. Species
that are unable to live independently of a host and are permanent parasites
are usually the most injurious to the animal. This is especially true of
parasites that are capable of puncturing the skin or burrowing into it.
Temporary parasites may cause fatal forms of disease. This is true of the
larva? of the sheep bot-fly, which develop in the sinuses of the head,
causing severe inflammation of these parts, nervous symptoms and death. The
character of the symptoms of a parasitic disease depends on the habits of
the parasite, and the tissue or organ, that it may attack.

The parasitic flies belong to the order _Diptera_, and the families
_Muscidae_ and _OEstridae_. Fleas belong to the sub-order _Pulicidae_. The
order _Hemiptera_ includes the lice, and the most important families are
_Pediculidae_ and _Ricinidae_. Mites and ticks belong to the order
_Acarina_. The most important parasites belonging to this order are the
_Sarcoptidae_ and _Ixodidae_.

OESTRIDAE.--The three common bot-flies are the _Gastrophilus equi_,
_Hypoderma lineata_ and _OEstrus ovis_. These flies are important because