In this order, which includes all gnat-like creatures, as well as those insects that are generally known as flies, the first pair of wings are well developed, while the second pair are rudimentary, and represented merely by a pair of scales, or by two little pin-like bodies called the balancers or _halteres_. Some are provided with piercing organs by means of which they can inflict a small wound and then extract the juices of their victim, as does the female gnat, but the majority have a proboscis adapted for suction only. The larvæ of the _Diptera_ are generally limbless maggots, gifted with a pair of jaws, and they are usually very voracious feeders, devouring decomposing animal or vegetable matter in enormous quantities. If we turn over a fermenting mass of the miscellaneous matter thrown up on the beach quite beyond the reach of the tides, we may observe a multitude of little maggots which feed on the moist, odorous portion that was protected from the direct rays of the sun, together with a number of dark-coloured pupæ that lie at the very bottom of the heap or buried in the sand below. These are two stages of the black fly (_Cœlopa frigida_) that is so attentive to us when we rest on the dry sand above high-water mark. This fly is very like the common house-fly in general appearance, though its body is rather smaller. Other species of the same genus often accompany them, all being very similar in general appearance and habits, and none of the larvæ seem adapted to a life in the water. They are always found beyond the reach of the tide, and are drowned if submerged for any length of time. Another species belonging to the genus _Actora_ will often be seen in the same company, and this is readily distinguished by their lighter greyish colour and its superior size. Also, along the water-line, we often meet with species of the family _Dolichopodidæ_, so called on account of the length of their legs, and noted for the beautiful metallic colours which adorn their bodies. These flies are carnivorous in habit, deriving their food from living as well as from freshly killed animals, and their short, fleshy proboscis contains a piercing bristle by which they can puncture the skins of the animals that provide them with food. Most of the flies of this group live on trees, walls, fences, &c., where they pursue and attack their prey, but certain species follow the line of breakers on the sea shore, as before indicated, and obtain their food from the various marine animals that are stranded on the beach. A peculiar feature of the family is the nature of the abdomen of the males, which is bent under the body and furnished with a number of appendages.