The larva of this little insect is parasitic on the useful larva of Syrphus ortas whose life-history is recorded on page [57]. It is very common in some instances, and must consequently destroy a considerable number. It entirely eats the soft portions of the insect, and may afterwards be found lying snugly within the hard empty shell of the deceased syrphus pupa, which acts as a cocoon for it while undergoing its own pupa state. The perfect insect may be often observed amongst herbage, searching for syrphus larvæ to deposit its eggs in.
CHAPTER V
The next Order which comes under review is the Diptera, which includes all the two-winged insects, and constitutes a most extensive Order in respect to the number of distinct species. When, however, the numbers of individuals of the same species are considered, it is probable that this Order includes a greater proportion of the insect-world than all the others put together. The preponderance of these insects over the rest holds good with greater force in New Zealand than in many other countries, and this fact may be almost inferred from the large number of spiders present here, which are chiefly dependent on Diptera for their support. The important function of clearing away refuse matter is almost entirely performed by the members of this Order, as the Necrophagous Coleoptera and other scavengers which exist in such large numbers in many countries are practically absent here, and their work consequently devolves upon dipterous insects.
Group Nemocera.
Family Culicidæ.
Culex iracundus (Plate [IV]., fig. 1, 1a larva, 1b pupa).
The mosquito is only too familiar to every one from its ceaseless attacks; it occurs almost everywhere, but is most abundant in marshy situations. The larva (Fig. 1a) inhabits all stagnant waters, where it may be found very abundantly throughout the summer, and when disturbed it plunges about with great agility. Its food consists of the numerous animalculæ swarming in all still waters during the greater portion of the year. These are captured by means of two curious anterior appendages, which are fringed with long hair, and pulled through the water like a fisherman's net; they are then withdrawn into the mouth and the contents devoured, the hungry insect again extending them for a fresh supply. These larvæ are generally seen suspended from the surface of the water by the curious air-tube which takes its rise from the penultimate segment of the abdomen, which is of considerable length. Its apex is armed with a row of stiff bristles, which effectually prevent the water from entering the spiracle there situated, so that the insect is enabled to respire when hanging from the surface, independently of any muscular action. It is also worthy of note that the intestine discharges itself into this tube, an arrangement which does not exist among the British species. After several moultings the transformation to the pupa state takes place. At this stage the insect (Fig. 1b) becomes much thickened anteriorly, this being the region of the head and thorax of the future gnat; all the limbs are easily detected on a close examination, as with lepidopterous pupæ. The upper portion is provided with two short appendages, fulfilling the same function as the air-tube of the larva, and which constantly support the pupa at the surface of the water. The terminal fins enable it to dash through the water with great rapidity when pursued by enemies; at other times it remains perfectly motionless, suspended from the surface of the water. It should be mentioned that none of these aquatic pupæ take any nourishment, neither have they any limbs properly so called. Their locomotion, although in some cases unquestionably rapid, is entirely effected by violent motions of the abdomen. I have been careful to point out these peculiarities as these animals have been regarded by many authors as active pupæ on a level with those of the Orthoptera and Hemiptera. This opinion, however, is manifestly erroneous; the pupæ of the nemocerous Diptera are on precisely the same footing as those of the Lepidoptera, and it would be almost as reasonable to call one of these active, because it wriggles out of its cocoon in the earth before the emergence of the moth. The perfect mosquito emerges from a rent in the thoracic shield of the pupa, drawing each pair of legs out separately, and placing them in front of it on the water; the wings and abdomen are then extracted and in a few moments it flies away.
The bites of these insects appear to distress some people much more than others, probably owing to constitutional differences. I should mention that the females alone engage in these attacks, the males being quite harmless and subsisting entirely on honey, which is doubtless the natural food of both sexes. The male and female mosquito are readily distinguished, the specimen figured belonging to the latter sex; her companion is chiefly remarkable for his plumed antennæ and beautiful palpi, which are very long and gracefully plumed. As many of the harmless insects which will be investigated are often mistaken for this species, and destroyed accordingly, I should like to advise my readers that they may at once distinguish all the venomous species of gnats by their long, lancet-like proboscis and loud humming noise during flight.
Closely allied to this insect is Culex argyropus, which might be called the coast mosquito as it is always found near the seashore, its larva living in brackish pools just above high-water mark. The perfect insect may be also seen skating along the surface of the water like a gerris[[6]]; it may be at once distinguished by its dark colour,.