1. Rearing. Larvae of Trombicula autumnalis were fed on young mice (about 8 days old); after 48 hr. at 25° C. they began dropping off fully engorged.

The post-larval stages were reared individually in separate filter-paper cells. At room temperature, 100% R. H., the engorged larvae were actively mobile for about 6–12 days, and for 12–20 days at 30° C., 100% R.H., before entering a quiescent stage (the developing nymphal stage). The nymphs emerged after about 33 days at room temperature, and 25 days at 30° C., from the time the engorged larvae dropped off the host.

Of the nymphs which developed to adults, four fed on a mixture of yeast, molasses, and agar plus chicken faeces; one fed upon the exuded contents of the eggs of Aëdes aegypti; and another fed upon a mixture of all the ingredients mentioned. Young and old nymphs of Trombicula autumnalis appeared to be incapable of feeding upon intact insects' eggs; neither did they show any particular predilection for the exuded contents of insects' eggs offered as a soft mass, and only with some difficulty were they made to introduce this food into the gut. When chicken faeces were offered as food the nymphs did not feed upon it.

A method of ‘forced feeding’ was adopted, that is to say, nymphs were placed upon or guided on to the periphery of the food mass provided. When a nymph moved away it was replaced upon the food, and the process was repeated until the nymph showed signs of introducing the food into the gut. Occasionally a nymph after being placed upon the food remained in situ and fed continuously for about 6–8 hr. until completely engorged.

2. The post-embryonic forms. The features of the stages (the larva nymph and adult), are described with respect to their development and growth.

The transition, or developing stages (the pre-nymph and pre-adult), are closed systems which resemble the pupal stage of a holometabolous insect. The developing mite is enclosed by the cuticular remains of the preceding mobile stage, and a second inner or intermediate cuticle. Large flattened cells of epidermal origin are present, usually lying against the inner surface of the intermediate cuticle. Emergence of the nymph and adult depends upon the combined effects of an active secretion, which causes a disintegration of the intermediate cuticle, and muscular exertion.

3.Dimorphism. Two kinds of dimorphism are shown during the growth of T. autumnalis; sexual dimorphism associated with size, and nymphal dimorphism depending on marked differences in body shape and in the length of the posterior setae.

Both kinds of dimorphism are initiated at the beginning of post-larval life (beginning of nymphal development).

The time which the engorged larva takes to settle is inversely proportional to the size of the post-larval stages; in other words, the amount of activity and accompanying usage of acquired food reserves before the engorged larva settles appear to govern the absolute size of the individual during post-larval life. There is an indication of small forms becoming male and larger ones female.

Two types of nymph (referred to in this paper as α-type and β-type nymphs) were bred from out-wardly identical larvae. The differences in the growth of the body as a whole and the posterior setae, which distinguish the two types of nymph, appear to be of developmental origin.

4. The life cycle in the natural environment. Laboratory findings confirm the view that only one generation is produced each year, but that successive broods are produced throughout the season of incidence.