(1) Blastothrix britannica, Gir., and Aphycus melanostomatus, Timb., are two important Chalcid parasites of the Scale Insect Lecanium capreæ.
(2) B. britannica passes through two generations in the year, and both males and females occur in approximately equal numbers. The first generation of adults are derived from hibernated larvæ, and emerge during May and early June. The female lays one or several eggs in the young fully grown host, only perforating the body-wall of the latter with her ovipositor and leaving the pedicel of the egg protruding to the exterior. The newly-hatched larva is unique among Hymenoptera in being metapnenstic, and its spiracular extremity remains attached to the chorion of the egg. By this means the parasite respires free air through the open apex of the pedicel. Subsequently it loses its attachment, becomes peripneustic with nine pairs of open spiracles, and lies free in the body-cavity of the Coccid. At this stage it frequently becomes enclosed in a phagocytic sheath formed by the host. Pupation takes place, within the body of the latter, and occurs towards the end of June; as many as forty-two pupze were found within a single Lecanium.
The second generation of adults emerge in greatest numbers during the first three weeks of July. The females utilise the very young larval hosts for purposes of oviposition, and lay a single egg within inch Coccid selected. The resulting larvæ pass through changes similar to those undergone in the first generation, but remain throughout the winter within the bodies of their hosts, and pupate, as a rule, during the following April. The Chalcids which emerge there from constitute the first generation of adults for that year.
(3) A. melanostomatus similarly passes through two animal generations, and the various stages of its life-history occur almost contemporaneously with those of the preceding species. Males, however, are less abundant than females, and occur in the approximate proportion of 1:3. The first generation of adults emerges between the beginning of May and the middle of June. The eggs are devoid of a pedicel, and are deposited within the body-cavity of the young adult hosts. The larvæ upon hatching are apneustic, respiration taking place through the skin. They subsequently become peripneustic with nine pairs of open spiracles, and are usually enclosed in a sheath or cyst. Pupation takes place within the host, and from one to forty-eight pupæ were found in a single example of the latter. The second generation of adults emerge about the same time as those of the previous species, and, similarly to the latter, they utilise the very young larval hosts for purposes of oviposition. The eggs are laid singly, a female never depositing more than one egg in an individual Coccid. The larval parasites over-winter in the apneustic condition, and give rise to the first generation of adults of the following year. A partial third generation of adults has been observed.
(4) The results of the first generation of parasitism upon the host are similar in both species of Chalcids. From the purely economic standpoint they are negligible. An average of about 53 per cent, of the Lecanium are attacked, but the latter do not succumb to the effects thereof until after they have deposited their ova. Furthermore, no conclusive evidence was discovered which might indicate any inhibitory action on the part of the parasitism in relation to egg production by the host. On the other hand, the effects of the second generation of parasitism are complete; about 40 per cent, of the hosts are attacked and destroyed a long period before attaining sexual maturity.
(5) The second generation of parasitism is of great importance in limiting the abundance of the host, which, in consequence, seldom occurs in sufficient numbers to constitute a pest, notwithstanding its high fecundity.
Memoirs: Observations on the Insect Parasites of Some Coccidæ