Important natural enemies of aphides and young caterpillars are the hover-flies, which can be easily recognised by their manner of flight. They are two-winged insects ([Fig. 14]c), and when on the wing hang motionless, as if suspended by some unseen means, to suddenly dart off with marvellous rapidity, until they hang motionless as before. These flies lay their eggs upon the foliage of plants infested by aphids or caterpillars, and from these eggs legless and headless larvæ emerge ([Fig. 14]d), and commence to search for and feed upon their victims.
Another important group of two-winged flies is the tachinids. They are rather robust, usually very bristly ([Fig. 14]e); they vary in size from that of a large blue-bottle to comparatively minute forms. The tachinids lay their eggs either upon their hosts or on the food plants of the latter, where they can be swallowed; some tachinids give birth to living larvæ, which crawl about in search of their victims.
Among the beneficial beetles are the well-known ladybirds ([Fig. 14]f); they are mostly oval in outline, dome-shaped above and flat below, while many of them are spotted by yellow, red, or white in a characteristic manner, though others are of one uniform colour. The eggs are laid on plants infested by the aphides and scale insects upon which the beetles and their larvæ ([Fig. 14]g) feed. There are other kinds of beetles of importance as predators, such as the common tiger-beetle, but they are not especially selective in their types of victims.
A very valuable group of insects includes the lace-wings or aphis-lions. The adult insects ([Fig. 14]h) carry the seemingly over-large lace-veined wings roof-like over the small body; the larvæ are alligator-like ([Fig. 14]i), and possess a pair of caliper-shaped jaws, by means of which they capture their prey. The eggs are laid directly on plants or are attached at the end of long stalks.
Birds.
It is generally recognised that birds are a very important aid in keeping destructive insects in check, though it is well-known that a great deal of damage can be done by these animals. Without a systematic study of the stomach contents of birds, it is not possible to decide when a species is beneficial or injurious, and in New Zealand no such study has been made; practically all the information we have is based on field observations, which are, unfortunately, influenced largely by the outlook of the observer, and are thus misleading. Though some species subsist for the most part on insects, most land-birds have a mixed diet of vegetable and animal food, but they specialise on an insect diet when rearing their young and when moulting.
FIGURE 14.