Most of the species may be recognised at once by their peculiar mode of flight, for they hover motionless in the air, and if alarmed dart off with a rapid motion which the eye cannot follow. _Milesia Crabroniformis_ is a very large species common in the south of Europe. About thirty species are found in the British Isles. Of these _Syrphus pyrastri_ is perhaps the best-known example. It is a very wasp-like creature, and is, indeed, mistaken by many people for a wasp. It is a very useful insect, feeding largely on aphides and plant lice, and should therefore be encouraged by every horticulturist. _Family 30, Conopidæ._--The _larvæ_ of most of these insects are parasitic on _Orthoptera_ and _Hymenoptera_. Many species have a resemblance to wasps in appearance, being striped with yellow and black. The typical genus, _Conops_, are slender flies measuring half an inch in length. _Family 31, Pipunculidæ._--These are black or brown insects. The head is large and round, generally broader than the thorax. The _larvæ_ are mostly parasitic on other insects. The flies may often be seen in great swarms in shady places in the vicinity of hedges, in lanes, etc. _Family 32, Platypezidæ._--This family contains many beautiful flies. Great numbers of them are marked with orange and silver and black. The _larvæ_ inhabit fungi. [Illustration: Fig. 145.--Eristalis Simili (Mag.)] _Family 33, Æstridæ._--The Bot Flies are well known on account of the annoyance they cause to cattle. They lay their eggs upon the hides of the animals, and the _larvæ_, when hatched, make their way under the skin, and there take up their abode, living on the juices of their unfortunate host. It is estimated that the loss occasioned by these troublesome pests amounts to millions of pounds in the British Isles alone. The best-known species is probably _Æstrus bovis_, which infests the ox. The cattle are so well aware of the danger attending the presence of this insect, that as soon as it appears near them, the whole herd exhibits the most unmistakable signs of terror, rushing about their pasture with their tails in the air, and in case of need taking refuge in water, where the fly will not follow them. Miss E. A. Ormerod has recently paid much attention to the economy of this insect, with the view to finding some remedy for its attacks. [Illustration: Fig. 146.--Nemestrina Longirostris (Mag.)] _Family 34, Muscidæ._--The _Muscidæ_ is the most numerous family, of the _Diptera_ and it includes many familiar insects. It is divided into two sections, containing thirteen sub-families. The first section, _Calypteræ_, contains five sub-families. The _Tachininæ_ are parasitic in the larval stage on _Lepidoptera_. The largest of the _Muscidæ_ found in Europe is _Echinomyria grossa_, which measures about three-quarters of an inch in length. [Illustration: Fig. 147.--Volucella Zonaria.] [Illustration: Fig. 148.--Conops Flavipes (Mag.)] [Illustration: Fig. 149.--Myopa Ferruginea (Mag.)]