1. Larvae of L. cuprina and Ch. rufifacies are capable of liquefying and digesting protein media without the intervention of bacteria. Both species and L. sericata secrete tryptic and peptic enzymes and at least L. cuprina does so from the moment of hatching. Tryptase is more abundant than peptase. Predatory activity, though a normal habit of Ch. rufifacies, is not necessary for any of these species and did not occur in the masses of Lucilia larvae used for extraction of the enzymes, although they were kept for periods up to 24 hours without food.
2. Partial development of larvae occurred in sheep dung, faeces-stained wool and in the products of keratin hydrolysis. Complete development took place in wool containing a "gummy crust" of dried exudate and in some samples of faeces-stained wool. Some of the samples of faeces-stained wool and those containing a "crust" have been demonstrated serologically by Dr I. M. Mackerras to have an increased content of soluble sheep protein as compared with normal wool. A marked increase of soluble protein has been demonstrated in struck wool both chemically and serologically.
3. Moisture, warmth, shelter and aeration are essential physical conditions for larval development. In addition, an alkaline reaction is relatively favourable and an acid reaction relatively unfavourable.
4. There are normally two stages in the development of a primary strike, the first stage being from hatching up to the time the larvae attack the skin, the outer layer or epidermis of which is approximately 36µ thick (Whitnall, 1931). During this stage they must feed, if at all, on materials already present. The second stage is from the commencement of an actual skin lesion up to full development of the maggots. During this stage there is a more or less copious serous exudation, which has been shown to be an adequate food for the full development of the maggots. Faeces-staining, presence of exudate due to a prior lesion, and products of wool hydrolysis have been shown to be adequate to carry the larvae through the first stage. Wool hydrolysis on the living sheep is probably not an important factor, judging by an examination of wool samples, but we have isolated organisms which, when growing on a nutrient medium, are capable of disintegrating wool fibre.
5. The rôle of bacterial activity in strike is complex and appears to be substantially as follows:
(a) to produce substances which attract the flies and stimulate them to oviposit;
(b) to provide food for the initial growth of the maggots, either by rendering assimilable the inert proteins, or by causing a skin reaction with a serous exudation.
6. The immediate work for the future is a more exact determination of:
(a) the nature of the food of the larvae in the early stage of growth on the living sheep;
(b) the factors which influence its production; and
(c) the mechanism by which the larvae invade the skin.
Factors Influencing the Action Point of the Collision Avoidance Manoeuvre
