Fig. 222—Under surface of the larva of Curupira (Paltostoma) torrentium, showing the suckers along the middle of the body, much magnified. Brazil. (After Fritz Müller.)
The metamorphoses of the European Liponeura brevirostris have been partially examined by Dewitz, who found the Insects in the valley of the Ocker in September.[[376]] He does not consider the "cephalothorax" to include an abdominal segment; and he found that two little, horn-like projections from the thorax of the pupa are really each four-leaved. The pupa is formed within the larval skin, but the latter is subsequently cast so that the pupa is exposed; its dorsal region is horny, but the under surface, by which it clings firmly to the stones of the rapid brook, is white and scarcely chitinised, and Dewitz considers that the chitinous exudation from this part is used as a means of fastening the pupa to the stones. Blepharoceridae possess, in common with Culex, Psychoda and Ptychoptera, the peculiar number of five Malpighian tubes, and it has been proposed by Müller to form these Insects into a group called Pentanephria.
Fam. 4. Culicidae (Mosquitoes, Gnats).—Antennae with whorls of hair or plumes, which may be very dense and long in the male, though scanty in the female; head with a long, projecting proboscis. Although there are few Insects more often referred to in general literature than Mosquitoes, yet the ideas in vogue about them are of the vaguest character. The following are the chief points to be borne in mind as to the prevalence of Mosquitoes:—The gently humming Gnat that settles on us in our apartments, and then bites us, is a Mosquito; there are a large number of species of Mosquitoes; in some countries many in one locality; in Britain we have ten or a dozen; notwithstanding the multiplicity of species, certain Mosquitoes are very widely diffused; the larvae are all aquatic, and specially frequent stagnant or quiet pools; they are probably diffused by means of the water in ships, it being known that Mosquitoes were introduced for the first time to the Hawaiian Islands by a sailing vessel about the year 1828. Hence it is impossible to say what species the Mosquitoes of a given locality may be without a critical examination. No satisfactory work on the Mosquitoes of the world exists. Urich states that he is acquainted with at least ten species in Trinidad. The species common in our apartments in Central and Southern England is Culex pipiens, Linn., and this species is very widely distributed, being indeed one of the troublesome Mosquitoes of East India. The term Mosquito is a Spanish or Portuguese diminutive of Mosca. It is applied to a variety of small flies of other families than Culicidae, but should be restricted to these latter. The irritation occasioned by the bites of Mosquitoes varies according to several circumstances, viz. the condition of the biter, the condition or constitution of the bitten, and also the species of Mosquito. Réaumur and others believed that some irritating fluid is injected by the Mosquito when it bites. But why should it want to irritate as well as to bite? Macloskie, considering that the Mosquito is really a feeder on plant-substances, suggests that the fluid injected may be for the purpose of preventing coagulation of the plant-juices during the process of suction. It is a rule that only the female Mosquito bites, the male being an inoffensive creature, and provided with less effectual mouth-organs; it has, however, been stated by various authors that male Mosquitoes do occasionally bite. It is difficult to understand the blood-sucking propensities of these Insects; we have already stated that it is only the females that suck blood. There is reason to suppose that it is an acquired habit; and it would appear that the food so obtained is not essential to their existence. It has indeed been asserted that the act is frequently attended with fatal consequences to the individual that does it. The proper method of mitigating their nuisance is to examine the stagnant waters in localities where they occur, and deal with them so as to destroy the larvae. These little creatures are remarkable from the heads and thorax being larger and more distinct than in other Dipterous larvae. Their metamorphoses have been frequently described, and recently the numerous interesting points connected with their life-histories have been admirably portrayed by Professor Miall,[[377]] in an accessible form, so that it is unnecessary for us to deal with them. Corethra is placed in Culicidae, but the larva differs totally from that of Culex; it is predaceous in habits, is very transparent, has only an imperfect tracheal system, without spiracles, and has two pairs of air-sacs (perhaps we should rather say pigmented structures possibly for aerostatic purposes, but not suppliers of oxygen). The kungu cake mentioned by Livingstone as used on Lake Nyassa is made from an Insect which occurs in profusion there, and is compressed into biscuit form. It is believed to be a Corethra. One of the peculiarities of this family is the prevalence of scales on various parts of the body, and even on the wings: the scales are essentially similar to those of Lepidoptera. Though Mosquitoes are generally obscure plain Insects, there are some—in the South American genus Megarrhina—that are elegant, beautifully adorned creatures. Swarms of various species of Culicidae, consisting sometimes of almost incalculable numbers of individuals, occur in various parts of the world; one in New Zealand is recorded as having been three-quarters of a mile long, twenty feet high, and eighteen inches thick. There is good reason for supposing that Mosquitoes may act as disseminators of disease, but there is no certain evidence on the subject. The minute Filaria that occurs in great numbers in some patients, is found in the human body only in the embryonic and adult conditions. Manson considers that the intermediate stages are passed in the bodies of certain Mosquitoes.[[378]]
Fam. 5. Chironomidae (Gnats, Midges).—Small or minute flies of slender form, with narrow wings, without projecting rostrum, usually with densely feathered antennae in the male, and long slender legs. The flies of this family bear a great general resemblance to the Culicidae. They are much more numerous in species, and it is not improbable that we have in this country 200 species of the genus Chironomus alone. They occur in enormous numbers, and frequently form dancing swarms in the neighbourhood of the waters they live in. The species are frequently extremely similar to one another, though distinguished by good characters; they are numerous about Cambridge. Many of them have the habit of using the front legs as feelers rather than as means of support or locomotion. This is the opposite of what occurs in Culicidae, where many of the species have a habit of holding up the hind legs as if they were feelers. The eggs of Chironomus are deposited as strings surrounded by mucus, and are many of them so transparent that the development of the embryo can be directly observed with the aid of the microscope. They are said to possess a pair of air-sacs. The larvae, when born, are aquatic in habits, and are destitute of tracheal system. They subsequently differ greatly from the larvae of Culex, inasmuch as the tracheal system that develops is quite closed, and in some cases remains rudimentary. There is, however, much diversity in the larvae and also in the pupae. The little Blood-worms, very common in many stagnant and dirty waters, and used by anglers as bait, are larvae of Chironomus. They are said to be αἱ Ἐμπίδες of Aristotle. The red colour of these larvae is due to haemoglobin, a substance which has the power of attracting and storing oxygen, and giving it off to the tissues as they require it. Such larvae are able to live in burrows they construct amongst the mud. Some of them, provided plentifully with haemoglobin, are in consequence able to live at great depths, it is said even at 1000 feet in Lake Superior, and come to the surface only occasionally. A few are able even to tolerate salt water, and have been fished up from considerable depths in the sea. It is a remarkable fact that these physiological capacities differ greatly within the limits of the one genus, Chironomus, for some of these species are destitute of haemoglobin, and have to live near the surface of the water; these have a superior development of the tracheal system. The pupae of Chironomus have the legs coiled, and the thorax, instead of being provided with the pair of tubes or trumpets for breathing that is so common in this division of Diptera, have a pair of large tufts of hair-like filaments.[[379]] A very curious form of parthenogenesis has been described by Grimm[[380]] as existing in an undetermined species of Chironomus, inasmuch as the pupa deposits eggs. Although this form of parthenogenesis is of much interest, it is not in any way to be compared with the case, already referred to, of Miastor (p. [461]). The "pupa" is at the time of oviposition practically the imago still covered by the pupal integument; indeed Grimm informs us that in some cases, after depositing a small number of ova, the pupa became an imago. This parthenogenesis only occurs in the spring-generation; in the autumn the development goes on in the natural manner. The case is scarcely entitled to be considered as one of paedogenesis.
Gnats of this family, and believed to be a variety of Chironomus plumosus, are subject to a curious condition, inasmuch as individuals sometimes become luminous or "phosphorescent"; this has been noticed more specially in Eastern Europe and Western Asia. The whole of the body and legs may exhibit the luminous condition, but not the wings. It has been suggested by Schmidt that this condition is a disease due to bacteria in the body of the gnat.[[381]]
Ceratopogon is a very extensive genus, and is to some extent anomalous as a member of Chironomidae. The larvae exhibit considerable variety of form. Some of them are aquatic in habits, but the great majority are terrestrial, frequenting trees, etc. The former larvae are very slender, and move after the manner of leeches; they give rise to imagos with naked wings, while the terrestrial larvae produce flies with hairy wings. There are also important distinctions in the pupae of the two kinds; the correlation between the habits, and the distinctions above referred to, is, however, far from being absolutely constant.[[382]] Certain species of midges are in this country amongst the most annoying of Insects; being of very minute size, scarcely visible, they settle on the exposed parts of the body in great numbers, and by sucking blood create an intolerable irritation. Ceratopogon varius is one of the most persistent of these annoyers in Scotland, where this form of pest is much worse than it is in England; in Cambridgeshire, according to Mr. G. H. Verrall, the two troublesome midges are the females of C. pulicaris and C. bipunctatus.
Fam. 6. Orphnephilidae.—Small, brown or yellowish flies, bare of pubescence, with very large eyes contiguous in both sexes, and with antennae composed of two joints and a terminal bristle; both the second joint and the bristle are, however, really complex. One of the smallest and least known of the families of Diptera, and said to be one of the most difficult to classify. The nervures of the wings are very distinct. Nothing is known of the habits and metamorphoses; there is only one genus—Orphnephila; it is widely distributed; we have one species in Britain.
Fam. 7. Psychodidae (Moth-flies).—Extremely small, helpless flies, usually with thickish antennae, bearing much hair, with wings broader than is usual in small flies, and also densely clothed with hair, giving rise to a pattern more or less vague. These flies are very fragile creatures, and are probably numerous in species. In Britain forty or fifty species have been recognised.[[383]] A South European form is a blood-sucker, and has received the appropriate name of Phlebotomus. The life-history of Pericoma canescens has recently been studied by Professor Miall.[[384]] The larva is of aquatic habits, but is amphibious, being capable of existing in the air; it has a pair of anterior spiracles, by means of which it breathes in the air, and a pair at the posterior extremity of the body, surrounded by four ciliated processes, with which it forms a sort of cup for holding air when it is in the water. The favourite position is amongst the filaments of green algae on which it feeds. A much more extraordinary form of larva from South America, doubtless belonging to this family, has recently been portrayed by Fritz Müller, under the name of Maruina.[[385]] These larvae live in rapid waters in company with those of the genus Curupira, and like the latter are provided with a series of suctorial ventral discs. Fritz Müller's larvae belong to several species, and probably to more than one genus, and the respiratory apparatus at the extremity of the body exhibits considerable diversity among them.
Fam. 8. Dixidae.—The genus Dixa must, it appears, form a distinct family allying the Culicid series of families to the Tipulidae. The species are small, gnat-like Insects, fond of damp places in forests. We have four British species (D. maculata, D. nebulosa, D. aestivalis, D. aprilina). The genus is very widely distributed, occurring even in Australia. The larvae are aquatic, and have been described by Réaumur, Miall, and Meinert. The pupa has the legs coiled as in the Culicidae.