Leptus autumnalis, Shaw, 1790.
Fig. 349.—Leptus autumnalis, with so-called sucking proboscis. Enlarged. (After Gudden.)
Leptus occur as parasites in the human skin and cause a cutaneous disease known as autumn erythema, and produce a very unpleasant sensation on account of the troublesome itching; in children it is very often accompanied by fever.[332]
Formerly these mites were considered adult forms, but when they were recognized as mite larvæ they were taken for those of the spider-mite (Tetranychus telarius); the investigations of Hanstein, however, showed this to be a mistake. When Henking first investigated the development of Trombidium fuliginosum, parasitic in the larval stage on vine-fretters, he demonstrated the occurrence of a form very similar to Leptus autumnalis, and the “autumn, grass, or gooseberry” louse was commonly designated the Trombidium larva. Even before Henking’s work it had been described by Mégnin as the larva of Trombidium holosericeum, a red-coloured species, frequently occurring in spring and summer on the ground, trees, etc. This assumption, however, as Moniez was the first to explain, is not correct; indeed, as many as three species come under consideration: T. gymnopterosum, L., T. fuliginosum, Herm. (according to Brucker), and two species known hitherto only in the early stage, T. striaticeps, Helm. et Oudem., and T. poriceps, Helm. et Oudem., which are not only parasitic on mammals, but on birds, on Arthropods and especially on insects. Arthropods appear to be the normal hosts for the larvæ.
Fig. 350.—Leptus autumnalis: the so-called proboscis is formed around the hypopharynx sunk into the skin. 100/1. (After Trouessart.)
The above-mentioned forms invade the skin of man by means of their oral apparatus, by preference invading the orifices of the sebaceous glands so as to suck the blood; around the point attacked there arises a wheal about the size of a lentil, and around the inserted hypopharynx a fibrinous secretion, the “proboscis,” which, however, is a product of the host, just as chitinous secretions are provoked by Trombidia parasitic on Arthropods.