Fig. 288—Scale-Insect. A, Aspidiotus camelliae, on the stem of a plant; B, a female scale magnified. (After Green.)
Fam. 9. Coccidae (Scale-Insects, Mealy-bugs).—Insects, usually minute, with only a single claw to the foot; the male with one pair of wings, but without mouth-parts; the female wingless and usually so degraded in form that most of the external organs and appendages cannot be distinguished. The form in which these Insects are most generally known is that of a small scale or shell-like body closely adhering to leaves, fruits, or bark. The scales are of the most varied form, so that no general description can be given of them. The scale may be defined as an accumulation of excreted matter, combined with the cast skin or skins of the Insect, covering the body either totally or partially, and thus acting as a shield under which the subsequent development takes place. All Coccidae do not form scales; but the habit of excreting a large quantity of peculiar matters to the outside of the body is universal; this excreted substance is frequently white, and of a powdery nature, and Coccids of this kind are known as mealy-bugs. In other cases the exudation is like shell or glass, and the creature may become quite encysted therein. In this way the forms of Cocidae known as "ground-pearls" are formed. When first hatched from the egg Coccidae are mite-like creatures, and it is only subsequently that the females lose the power of locomotion. The females of numerous forms of Coccidae—more particularly the mealy-bugs—do not lose the antennae and legs. There is also a group (Brachyscelides) of Coccids that live in galls. This highly aberrant group is, however, peculiar to Australia; elsewhere very few gall-making Coccids have been discovered.
Fig. 289—Dactylopius longispinus. Female on portion of a fig-leaf. (After Berlese.)
There are upwards of 800 species of Coccidae at present known.[[534]] The family was monographed by Signoret about twenty-five years ago, and since then there has been very much matter concerning them published in a scattered manner.[[535]] No general work has been published on the British species, but Mr. Newstead is preparing one. The classification of Insects so minute as Coccidae, and with such extreme difference in the sexes, is, of course, a matter of great difficulty; the best divisions are those given by Green in his Coccidae of Ceylon.[[536]]
The fact that there is only one pair of wings in the perfect male Coccid would appear to ally these Insects with the Diptera; these Coccidae have, too, like the Diptera, a small appendage on each side of the metathorax. Witlaczil shows that these little processes may really represent a pair of wings, inasmuch as they are developed from imperfect folds of hypodermis, i.e. imaginal discs. Beyond these facts and the occurrence in certain females (Margarodes) of a great histolysis during the post-embryonic development, there is nothing to indicate any relationship between Coccidae and Diptera. It has been shown by Riley that these little processes, in some forms, serve as hooks to attach or control the true wings, and this function is never assumed by the halteres of Diptera. Although Coccidae are placed next Aphidae, yet the two families appear to be really very different. The modes of reproduction so peculiar in Aphidae reappear to a certain extent in Coccidae, but are associated with profound distinctions. Though the viviparous method of reproduction and parthenogenesis occur in Coccidae, yet they are only exceptional, and they are not put to the same uses by the species that exhibit the phenomena. Thus we have seen that in Aphidae generations of imperfect individuals are produced with rapidity, while the individual is not directly very prolific. In Coccidae the reverse is the case—the generations are usually similar to one another; they do not, as a rule, follow with rapidity, and the female is usually very prolific, thousands of young being sometimes produced by a single individual. The extraordinary polymorphism of the species of Aphidae is not exhibited by Coccidae, though, contrary to what we find in Aphidae, the males and females are usually excessively different. The two families apparently also differ in that Coccidae are specially characteristic of warm climates, Aphidae of the temperate regions.
Parthenogenesis.—Owing to the fact that the males are very minute creatures, totally different from the females, and living but a very short time, they were but little known to the earlier observers. It was therefore only natural to suppose that parthenogenesis was very common. Of late years the males of a great many species have become known, so that ordinary sexual reproduction must be considered as the normal method in Coccidae, although, in the great majority of cases, the male is still unknown. It has, however, been shown in numerous cases that parthenogenesis may occur even when males exist; and there are some abundant species of which it has not been possible to find a male. In 1887 Moniez[[537]] announced that he had discovered the male of Lecanium hesperidum (one of the notoriously parthenogenetic species) in an ovarian cul-de-sac in the body of the female, and he therefore considers that sexual reproduction occurs. He does not say how pairing takes place, and we are not aware that his observation has been confirmed. If correct it will be necessary to reconsider the whole question as to parthenogenesis in Coccidae. Apterous males are known in two or three species.
The post-embryonic development of Coccidae is of the most unusual character. It is quite different in the two sexes, and in each of them it presents features not found elsewhere. It has, however, as yet been studied in only a few forms, and even in them is incompletely known.
Fig. 290—Instars of Dactylopius citri. (After Berlese.) A, Egg; B, young larva; C, first male nymph; D, second male nymph; E, adult male; F, adult female. All equally magnified. x 20.