It is known that the females of some species of parasites can distinguish hosts that have already been parasitised and refrain from ovipositing in them. A study was made of the extent to which Spalangia drosophilae Ashm. avoids superparasitism of the puparia of Drosophila melanogaster Mg. by means of these two steps, which are termed discrimination and restraint, respectively.

Host puparia 0–24 hours old were stuck on paper, with water, equidistant ¼ in. from each other and placed in 4½ in. × 1 in. glass vials into which females of Spalangia were introduced for different periods. A series of experiments was set up, in which 25, 10 and 5 puparia were exposed to attack by 1 unmated or 1, 2 and 5 mated parasites for 24, 48 and 72 hours. In this way a wide range was obtained of the parasite-host (P/H) ratio, computed as the ratio of the number of female parasites, multiplied by the number of 24-hour periods for which they were used, to the number of host puparia exposed. Each treatment was replicated ten times, thus entailing a total of 360 experiments. The sequence of events associated with oviposition consisted of examination of the puparium by the females, stinging, followed sometimes by feeding on the fluid that exuded, and finally, but not invariably, oviposition. At the end of the experiment each puparium was examined and the number of eggs present, or the occurrence of stinging only, was recorded. The chance of a puparium being attacked was unaffected by its position on the paper and there was no significant difference between the total number of eggs that virgin and mated females laid under the same conditions.

The results were assessed by comparing the observed distribution of parasite eggs amongst host puparia with the probable random distribution calculated from a formula that is given. The divergence between these distributions was greatest at the lower P/H values, and very little superparasitism occurred unless these exceeded 3/25, at which point total parasitism reached about 60–70 per cent. As the P/H value increased, so did the total parasitism and the degree of superparasitism, and the deviation of the observed from the random distribution of eggs diminished, although even amongst experiments in which parasitism reached 91–100 per cent., superparasitism was still avoided in 15·6 per cent, of them.

The degree to which females of Spalangia can restrain themselves from ovipositing in hosts that have been parasitised was estimated by plotting the percentage of experiments in which restraint was observed to break down, against the level of attack, and comparing the distribution of the points so obtained with the curves that would relate these two functions if the power of restraint was to break down when the female was confronted with 2, 3 or 4 successive puparia that had already been attacked. It is concluded that the assumption that restraint breaks down at about the third successive unsuitable encounter gives the best fit with the observed data.

Contributory evidence that females of Spalangia can distinguish parasitised hosts was obtained by direct observation of their behaviour, which indicated that, on the average, females took about 3½ minutes to examine an unparasitised host before deciding whether to attack or not, but, in the case of hosts already parasitised, either rejected them in half that time, or only accepted them after unduly prolonged examination.

When individual mated females were supplied daily for 16 days with a single puparium, then for 3 days with 25 puparia, then again for 8 days with a single puparium and finally for 8 days with 25 puparia, the number of eggs laid daily was restricted to less than two when only a single host was available, but rose to about 9–11 on the first day an excess of hosts was provided and then fell to a normal output. The females thus adapt their egg-laying to some extent to the number of hosts available.

It was hoped that this elucidation of the combination of perfect discrimination but imperfect restraint exhibited by Spalangia. when ovipositing, together with the data previously obtained on its biology, would make it possible to predict the fluctuations of a population of Spalangia in the field and its effect on the natural host, Oscinella frit (L.), but the factors involved, and their interactions, are too complex, particularly when weather effects are considered, and it is concluded that any predictions of population must be deduced empirically from field observations over a number of years.