The Congo Floor Maggot, Auchmeromyia luteola (F.), in a Laboratory Culture

Extract

An account is given of the life-history of Auchmeromyia luteola in a laboratory culture maintained in London for two-and-a-half years. Attention was centred on the bloodsucking larva, known as the Congo Floor Maggot, which is an intermittent ectoparasite specific to man. It was reared chiefly on the natural host, but a strain has been maintained on shorn guinea-pigs through several generations. It was also found possible to rear the larva on free blood.

The known distribution of A. luteola is reviewed on the basis of published records, museum collections and information from scientists in Africa. The species is highly successful in both the wettest and the driest parts of the Ethiopian region, but does not seem to extend south of Durban. It can flourish only where man occupies permanent settlements and makes his bed on the floor within reach of the maggot. Strains originating in Nyasaland, in the western part of the Belgian Congo, and in the Anglo-Egyptian Sudan were successfully cross-mated and produced fertile eggs. The second generation from these crosses, however, was not always fully fertile.

The method of cultivation of the material is described.

Batches of eggs hatched 36–60 hours after oviposition when kept at 26–28°C. and 50–50 per cent. R.H. In drier atmosphere development was delayed and took from 3 to 7 days at 23°C. and 10 per cent. R.H.

The habits of the larvae are discussed and it is shown that larvae took 20 minutes to gorge (although newly hatched specimens often stopped feeding after 10 minutes). Given the opportunity, a meal was taken daily except for a day missed before each moult. No larva could be induced to bite twice in one day unless the first feed had been interrupted.

The rate of growth of the larvae was found to be strongly influenced by the feeding schedule, those receiving. 4 meals a week having a much higher rate than those receiving only 2. There was also a correlation between rate of growth and temperature but the correlation with relative humidity was less well marked. After fasts of several days, meals of the order of two and a half times their own weight were taken by larvae of all three instars.

Larvae which were fed four times a week and kept at 23°C., besides growing faster than others fed at less frequent intervals, reached higher maximum and pupal weights and started to pupate on the 26th day. Larvae fed at less frequent intervals took larger meals but lighter pupae and flies were produced.

The time of moulting appeared to be directly related to weight and only through this to the environment. The first moult occurred after the larva had gorged to a weight of 1·5–2·1 mg. (usually at the 2nd or 3rd meal), the second moult after it had reached 12–19 mg. (at the 4th to 7th meal).

There are wide limits of weight between which the onset of pupation can occur, the upper limit being higher for female than male larvae. If the feeding schedule (in relation to climate) is favourable, the larvae reach a maximum weight which induces the onset of pupation regardless of other factors. If the meals are scarce, growth is retarded more than metabolism and time becomes the limiting factor; then, the scarcer the meals, the smaller, not the later, the pupae. If the meal schedule is so adverse that the larvae cannot reach the minimum weight for pupation (about 97·5 mg.) in the time set by their metabolic rate in the given climate, death ensues without pupation.

No larva has been known to complete an instar on a single meal. The minimum for complete development in any climate is probably six meals, two in each stage. Larvae reared at 28·5°C. in 60 per cent. R.H. on five meals a week, moulted after the rd and 7th meals and pupated after the 16th or 17th.

Failure to moult, followed by death, occurred in all strains in the laboratory. It is believed that this is sometimes due to overcrowding and sometimes to the larva being disturbed when ready to moult. Inability to moult was commonest in the dry atmospheres and among larvae fed only twice a week.

Saturated air was fatal to the larva but not to the pupa. The species tolerates a wider range of atmospheric humidity than most insects, and was even reared successfully in an atmosphere of 10 per cent. R.H.

Female larvae in the third instar take more blood than males and lose more by excretion ; they also grow larger and produce heavier pupae and adults.

The temperature lethal in one hour to larvae having completed the second moult is denned within about one degree (42·5–43·5°C.) and does not appear to vary according to atmospheric humidity.

The Floor Maggot can survive fasts perhaps longer than any other Dipterous larva. At 28·5°C. and 90 per cent. R.H. survival of first-instar larvae, unfed, after one meal and after two meals, was 9–20, 8–21 and 4–21 days respectively ; third-instar larvae after moult (i.e. at 12·19 mg.), and at about 90 mg. survived 17–18 and 28–47 days respectively. At the same temperature but 10 per cent. R.H. much more weight was lost and the survival time was much less. At 23°C. newly hatched unfed larvae survived 5–37 days at 60 per cent. R.H. and 9–22 days at 10 per cent. R.H., while third-stage larvae survived 25–48 days at 90 per cent R.H. and 9–19 days at 30 per cent. R.H. Newly hatched larvae lived for 3–8 days at 35°C. in 60 per cent. R.H.

The pupal stage lasts about 9 days at 34°C., 11 days at 28·5°C. and 15–16 days at 23°C. and is unrelated to atmospheric humidity. Losses in weight at different atmospheric humidities were studied; the proportion lost was about 16 per cent, at 90 per cent. R.H. and about 25 per cent, at 10 per cent. R.H.

The habits of the adult flies are discussed ; human faeces appear to be the staple diet. The male seeks the female persistently and mating is protracted and occurs repeatedly. One male can fertilise several females. Oviposition and development continue all the year round without diapause. In the laboratory at 22–23°C. a female normally laid about 54 eggs at her first oviposition, and in one case a female laid as many as 6 batches of fertile eggs. In warm weather, the first batch was laid about 16 days after emergence (or after 20–23 days at 23°C), smaller batches being laid subsequently at intervals of 5–8 days. The female lived in the laboratory up to 93 days and the male up to 85 days.

The life-cycle under natural conditions is roughly estimated as 10 weeks, so that five generations a year might be expected.