Encyclopædia Britannica, Ninth Edition/Weevil
WEEVIL, a very old Anglo-Saxon term, now commonly applied to the members of a group of Coleoptera termed the Rhyncophora (see vol. vi. p. 133). This group is characterized by the prolongation of the head into a rostrum or proboscis, at the end of which the mouth, with its appendages, is placed. The antennæ are short, usually elbowed, and often end in a club-shaped swelling. The basal portion of the antennæ frequently lies in a depression at the side of the rostrum, and this gives the antennæ the appearance of emerging half-way along the rostrum. The mouth appendages are small; the mandibles, however, are stout. The palps are very short and conical as a rule. The body is usually small; in shape it varies very much. The elytra are very hard, and in some cases fused with one another, rendering flight impossible. The larvæ are white, fleshy, apodal grubs, with a series of tubercles along each side of the body; the head is round, and bears strong jaws, and sometimes rudimentary ocelli. They are exclusively phytophagous. The Rhyncophora embrace three families,—(1) the Curculionidæ, or true weevils, (2) the Brenthidæ, and (3) the Bruchidæ.
The Curculionidæ form one of the largest families amongst the Coleoptera, the number of species described exceeding 10,000, arranged in 1150 genera. The antennæ are elbowed, and clavate, with the basal portion inserted in a groove. The third tarsal joint is generally bilobed. Over 400 species exist in Great Britain, few of which exceed half an inch in length. The genera Phyllobius and Polydrosus include some of the most beautiful insects found in Britain,—their brilliancy, like that of the Lepidoptera, being due to the presence of microscopic scales. The diamond beetle of South America, Entimus imperialis, is another singularly beautiful weevil; its colour is black, studded with spangles of golden green. The immense family of the Curculionidæ includes members which differ greatly from one another in size, colour, and appearance; even the rostrum, the most striking common characteristic, varies greatly. The form of the body is very various: some are rounded or oval, others elongated, almost linear; some are covered with warty protuberances, whilst others are smooth and shining, often with a metallic lustre.
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One of the commonest members of this family in Great Britain is the Nut Weevil, Balaninus nucum. It is of a brownish colour, varied with yellow, the legs reddish. Its rostrum is unusually long, being five-sixths of the body length in the female, and slightly shorter in the male. The antennæ are 7-jointed. The first three joints are much longer than thick; the four following are shorter, and the seventh not longer than thick. The larva is very common in hazel nuts and filberts. When the nuts are about half-grown, the female bores, with its rostrum, a minute hole in the still comparatively soft nut shell, and deposits an egg within the nut. The egg is said to be pushed in by means of the long rostrum. As the nut grows the slight puncture becomes almost obliterated, so that it is unnoticed by all but the most observant eye. The larva is a thick white grub with a brownish head, bearing fleshy tubercles along its side. It feeds upon the substance of the nut. The nuts which are infested by this insect are usually the first to fall to the ground; the larva then bores a round hole through the nut shell, by means of its jaws, and creeps out. It hides itself in the ground during the winter, and in the spring it passes into the pupa stage, from which it emerges about August as the full-grown insect. A nearly allied form, Balaninus glandium, attacks both hazel nuts and acorns.
In an unobtrusive way weevils do immense harm to vegetation. This is effected not so much by their numbers and their powers of consumption, as amongst caterpillars, but by their habits of attacking the essential parts of a plant, and causing by their injuries the death of the plant affected. They destroy the young buds, shoots, and fruits, and attack the young plants in their most delicate organs. Many of them devour seed, as the Corn Weevils, Calandra granaria (see Wheat) and C. oryzæ, and in this way vegetation is severely injured, and its spread seriously checked. Others cause much damage in forests, by boring under the bark and through the wood of trees, whilst some even burrow in the tissue of the leaves.
The Brenthidæ are by some authorities included in the family Curculionidæ. They include some 275 species, and are almost exclusively tropical. Their antennæ are 11-jointed, not elbowed. The rostrum is straight and very long. The shape of the body is very long and narrow, the first and second abdominal segment being very long. Allied to these is the sub-family Attclabides.
The Bruchidæ form a somewhat larger family than the foregoing, containing over 400 species. The antennæ are straight, and inserted upon the head just in front of the eyes; they are 11-jointed, and serrated or toothed in the inside. The rostrum is short. Bruchus pisi causes considerable damage to pease; during the spring the beetle lays its eggs in the young pea, which is devoured by the larva which hatches out in it.