1. While investigating the parasites of the holly leaf-miner (Phytomyza ilicis Curt.) with a view to utilizing them in the control of this troublesome pest of holly in western Canada, a species of Opius, which on examination proved to be new to science, was reared from the fly puparia.

2. A fairly complete account of the general systematics, distribution, biology, and morphology of the various developmental stages of this parasite is set down in the preceding pages. The primary larva is particularly interesting because of its unusual orientation. After the anatomical details had been worked out it was discovered that the concave side of the larva, which would normally be regarded as the ventral surface, is actually the dorsal one.

3. The genus Opius, whose distribution is world-wide, contains a very large number of species which parasitize important economic pests. In temperate regions the insects which suffer most from their attacks are species of Pegomyia, Agromyza, Rhagoletis, Phytomyza and Cerodonta, whilst in tropical and subtropical areas the most favoured hosts belong to one or other of the two genera Dacus and Anastrepha.

4. The host relationship of the genus, because of its importance from both economic and taxonomic standpoints, is discussed at some length.

5. In the first stadium Opius ilicis is a larval parasite, but the three succeeding instars live in the host pupa, and the imago emerges from the puparium. A very interesting phase in the life history of this parasite occurs towards the end of the first stage. At this point the development of the larva is arrested and further growth cannot take place until the host has pupated.

6. Very little work has so far been carried out on the larval morphology of the Opiinae, but that done up to the present, including the foregoing descriptions, would seem to indicate that the larvae of this tribe form a fairly homogeneous group. The main distinguishing characters of these larvae are listed in section VII of this paper.

7. It is pointed out that O. ilicis, in spite of being intrinsically inferior to Chrysocharis gemma, is responsible for the destruction of a certain number of hosts which escape the attentions of the latter parasite, and although the percentage accounted for is small (maximum parasitism in 1939 4%), it nevertheless fills a particular niche of its own, and so must be of some definite value in the scheme of control.

8. The chief method employed by the first instar of Chrysocharis gemma in the destruction of rival Opius larvae would appear to be direct mandibular attack. Several reasons have been put forward to account for the decided inferiority which is exhibited by the Braconid when it comes into conflict with this Chalcid.

9. In section X, a number of interesting points which have a general bearing on the study of parasite larvae are discussed. These include the cephalic skeleton and its probable function in successive instars, the taxonomic value of this structure in the parasitic Hymenoptera, the apparent absence of a tracheal system in the second and third instar larvae of O. ilicis, and arrested development in the Opiinae and some related forms.