Chapter VIII: Hemiptera, or Bugs—anoplura 532 (18)

FAM. 44. TORTRICIDAE.—Moths of small size, with a rather ample wing area, with the wing-fringes never as long as the wings are wide (long across), the hind wings without a pattern: the anterior nervure on the hind wings is simply divergent from that next to it, and the internal nervure, 1b, is very evidently forked at the base. The larvae inhabit their food, which may be rolled up or twisted leaves, or the interior of fruits and herbs, or galls, or even roots; they exhibit less diversity than is usual in other large series of moths; all have the normal complement of sixteen legs. This group is a very extensive one, but is much neglected owing to the great difficulties attending its study; it is not recognised in Hampson's Table of families given on p. 370, being there merged in Tineidae. It appears, however, to be a really natural group, and it is not desirable to merge it in the sufficiently enormous assemblage of the Tineidae till this has been shown to be necessary by the light of a greater knowledge of the external anatomy than we possess at present. The term Microlepidoptera is frequently met with in entomological literature, and should, we think, be confined to the two series Tortricidae and Tineidae. The Pterophoridae, and even the Pyralidae, have been, and still sometimes are, included under this term, but at present it seems best to limit its application as is here suggested.

Three great divisions are at present recognised; these were formerly called by Meyrick,[329] Tortricidae, Grapholithidae, Conchylidae; subsequently,[330] he has adopted the names Tortricidae, Epiblemidae, Phaloniadae. Lord Walsingham, who has devoted a great deal of time and study to the elucidation of this most difficult group, has suggested[331] that another change is desirable, and if so the nomenclature will be:—1. Tortricidae [or Tortricinae, according to the view that may be taken as to the group being family or sub-family]; 2. Phaloniidae [= the formerly used name, Conchylidae]; 3. Olethreutidae [= the formerly used name Grapholithinae = Epiblemidae, Meyr.]. We have upwards of 300 species in Britain, nearly 200 of which belong to the last division. The name Tortricidae refers to the habit the {428}larvae of these moths possess of rolling up leaves, or twisting and distorting shoots and buds.

The mode in which leaves and shoots are twisted and rolled by the very small larvae has been much discussed and is probably the result of two or three distinct causes:—1, the immediate operations of the larva; 2, the contraction of silk when drying; 3, changes in the mode of growth of the parts of the vegetable, resulting from the interference of the caterpillar. The larvae of this family that live in fruits are only too widely (we will not say well) known. Stainton gives as the habitat of _Epinotia funebrana_, "larva frequent in plum-pies"; the caterpillar of _Carpocapsa pomonella_ (the Codling-moth) mines in apples and pears, and its ravages are known only too well in widely distant parts of the world where fruit-trees of this kind are cultivated. _C. splendana_ lives in acorns and walnuts; _C. juliana_ in Spanish chestnuts. Two, if not more, larvae live in the seeds of Euphorbiaceous plants, and have become notorious under the name of jumping-beans, on account of the movements they cause. As these latter show no trace externally of being inhabited, the movements are supposed to be a mysterious property of the seed; they are really due to its containing a large cavity, extending, in one direction of the seed, nearly or quite from skin to skin; in this the larva makes a movement sufficient to alter the point of equilibrium of the quiescent seed, or as a free body to strike some part of it. The exact nature of the movements of the larva have not, we believe, been ascertained. There are, at least, two species of these Insects, and two plants harbouring them, known in the United States and Mexico, viz. _Carpocapsa saltitans_ living in the seeds of _Croton colliguaja_ and _Grapholitha sebastianiae_ living in the seeds of _Sebastiania bicapsularis_.

FAM. 45. TINEIDAE.—Small moths with the labial palpi more flexible and mobile than in other moths; usually separated and pointed. Hind wings frequently with very long fringes, the wing itself being proportionally reduced in size, and in consequence pointed at the tip. Larvae very diverse, almost always with habits of concealment. The series of forms included under this head is very numerous, the British species alone mounting up to 700, while the total described cannot be less than 4000. This number, however, must be but a fragment of what exists, if Mr. Meyrick be correct in supposing that a single one of the divisions of the family—Oecophoridae—comprises 2000 species in Australia and New Zealand alone.

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As the study of these Insects is attended with great difficulty on account of their fragility and the minute size of the great majority, it is not a matter for surprise that their classification is in a comparatively rudimentary state. We shall not, therefore, deal with it here. Neither can we attempt to give any idea of the extreme diversity in the colours, forms, and attitudes of these small Insects. The one shown in Fig. 207, is remarkable on account of the great accumulation of scales on the wings and legs. As regards the pointed wings and the long fringes, we may remark that it is probable that in many of these small forms the wings are passive agents in locomotion; a similar condition of the wings is found in other very minute Insects, _e.g._ Thysanoptera and Trichopterygidae; in all these cases the framework of the wings is nearly absent: in some forms of the Tineidae, _Opostega_, e.g. the nervules are reduced to three or four in each wing. The variety in habits is as great as that of the external form, and the larvae exceed in diversity those of any other group of Lepidoptera. No doubt a corresponding amount of diversity will be discovered in the details of structure of the perfect Insects, the anatomy of but few having been at present investigated. _Tinea pellionella_ has two very important peculiarities in its internal anatomy: the testes consist of four round follicles on each side, and, contrary to the condition generally prevalent in Lepidoptera, are not brought together in a common capsule: the two groups are, however, not quite free (as they are in _Hepialus_), but are connected by a loose tracheal network. Even more remarkable is the fact also pointed out by Cholodkovsky[332] that the adult Insect possesses only two Malpighian tubes instead of six, the normal number in Lepidoptera; in the larva there are, however, six elongate tubes. The group of forms to which {430}_Tinea_ belongs is remarkable for the diversity and exceptional character of the food-habits of the larvae; species subsist on dried camel's dung, various kinds of clothes, furs, and hair, and even about horns of deer and horses' hoofs: one species has been found in abundance in the hair of a live sloth, _Bradypus cuculliger_, under circumstances that render it possible that the larva feeds on the creature's hair, though it may feed on minute vegetable matter found in the hair. The larva of _Tinea vastella_ is occasionally found feeding on the horns of living antelopes. Several species of Tineidae are known to devour Scale-Insects.

_Lita solanella_ is notorious for the ravages it commits on stored potatoes. Quite a number of species live on cryptogamic matter, or in old wood; _Oinophila v-flavum_ feeds on the mould on the walls of cellars, and is reputed to be injurious by occasionally also attacking the corks of bottles containing wine. _Oecocecis guyonella_ is said to be the cause of galls on _Limoniastrum guyonianum_, a plant that, growing in the deserts to the south of Algeria, is a favourite food of camels, and is frequently entirely covered with sand. The deposition of an egg by this moth is believed by Guénée[333] to give rise to a gall in which the larva is entirely enclosed (like the larvae of the gall-flies). Of Clothes-moths there are at least three species widely distributed. _Trichophaga tapetzella_ is perhaps entitled to be considered the Clothes-moth; its caterpillar not only feeds on clothes, but spins webs and galleries amongst them. _Tinea pellionella_ is also very common; its larva lives in a portable case, while that of the third species, _Tineola biselliella_, forms neither a case nor definite galleries. We have found this the most destructive of the three at Cambridge. Clothes or valuable furs may be completely protected by wrapping them in good sound paper in such a way that no crevices are left at the places where the edges of the paper meet. Garments that have become infested may be entirely cleared by free exposure to air and sunshine.

Two species of _Tinea_ have been recorded as viviparous, viz. _Tinea vivipara_ in Australia, and an undetermined species in South America. The species of the genus _Solenobia_—in which the female is apterous—are frequently parthenogenetic. The group Taleporiidae, to which this genus belongs, is by some {431}classified with Psychidae, in which family, as we have pointed out, one or two parthenogenetic forms are also known.

The larvae of Tineidae, though they do not exhibit the remarkable armature found in so many of the larger caterpillars, are exceedingly diverse.[334] Some are entirely destitute of feet (_Phyllocnistis_). Others are destitute of the thoracic legs; _Nepticula_ is in this case, but it is provided with an increased number of abdominal feet, in the form of more or less imperfect ventral processes. Some mine in leaves, others live in portable cases of various forms. Some are leaf-miners during their early life, and subsequently change their habits by constructing a portable case. The genus _Coleophora_ affords numerous instances of this mode of life; the habits of these case-bearers exhibit considerable variety, and there are many points of interest in their life-histories. Change of habit during the larval life has already been alluded to as occurring in many Lepidoptera and is nowhere more strikingly exemplified than in certain Tineidae. Meyrick mentions the following case as occurring in an Australian Insect, _Nematobola orthotricha_;[335] the larva, until two-thirds grown, is without feet, and is almost colourless, and mines in the leaves of _Persoonia lanceolata_; but when two-thirds grown it acquires sixteen feet, changes colour, becoming very variegate, and feeds externally, unprotected, on the leaves. The cases of the case-bearing Tineids are usually of small size, and do not attract attention like those of Psychidae. A very remarkable one was discovered by Mr. E. E. Green in Ceylon, and was at first believed to be formed by a Caddis-worm. It has now been ascertained that the Insect forming it is the caterpillar of _Pseudodoxia limulus_, a Tineid moth of the group Depressariidae;[336] the case is composed of minute fragments of moss, sand, and lichens; the anterior end is dilated into a shield-like hood that covers and protects the anterior parts of the larva when feeding; the food is mosses and lichens on rocks and trees. Before pupating, the larva folds down the edges of the hood over the mouth of the tube, like an envelope, fastening them with silk. The case is fixed to the rock or other support and hangs there until the moth appears.

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The family Prodoxidae consists of some Tineids, the larvae of which feed in the pods and stems of the Yuccas of south-western North America; they have the mouth of very unusual form (Fig. 208, E), and some of them, by aid of this peculiar mouth, exhibit a remarkable modification of instinct. The facts are chiefly known from the observations of Riley[337] on _Pronuba yuccasella_, a moth living on _Yucca filamentosa_; this plant has been introduced into our gardens in this country, where it never, we believe, produces seed. The Yuccas are not fitted for self-fertilisation or for fertilisation by Insect agency of an ordinary kind. The progeny of the moth develops in the pods of the plant, and as these cannot grow until the flowers have been fertilised, the moth has the habit of fertilising the flowers at the time she lays her egg in the part that is to develop into the pod, and to be the food for her own progeny. The female moth first visits the stamens, and collects, by the aid of the {433}maxillae (which in this sex are very remarkably formed),[338] a considerable mass of pollen, which she holds by means of the peculiar maxillary tentacles; she then lays an egg in the pistil, usually of some flower other than that from which she has gathered the pollen; and after she has accomplished this act she carefully applies the pollen she had previously collected to the pistil, so as to secure the fertilisation of the flower and the development of the pod.

The species of _Prodoxus_ stand in a very peculiar relation to _Pronuba_. They also live in Yuccas, and have habits similar to those of _Pronuba_, with the important exception that, being destitute of the requisite apparatus, they do not fertilise the Yucca-flowers, and are thus dependent on _Pronuba_ for the steps being taken that are necessary for the rearing of the progeny of the two kinds of moth. Hence the name of Yucca-moth has been bestowed on _Pronuba_, and that of "bogus Yucca-moth" on the _Prodoxus_. The _Pronuba_ we figure is the largest and most remarkable species of the genus and fertilises _Yucca brevifolia_; the larva is destitute of abdominal feet, and in the pupa the spines on the back that exist in nearly all pupae that live in stems are developed to an extraordinary extent. The Yuccas do not flower every year, and the Prodoxidae have a corresponding uncertainty as to their periods of appearance, passing sometimes a year or two longer than usual in the pupal stage.

FAM. 46. ERIOCEPHALIDAE.—This family has recently been proposed for some of the moths formerly included in the genus _Micropteryx_.[339] They are small, brilliant, metallic Insects, of diurnal habits, but are very rarely seen on the wing, and it is doubtful whether they can fly much. These little Insects are of peculiar interest, inasmuch as they differ from the great majority of the Lepidoptera in at least two very important points, viz. the structure of the wings and of the mouth-parts. The mouth shows that we may consider that the Lepidoptera belong to the mandibulate Insects, although in the great majority of them the mandibles in the final instar are insignificant, functionless structures, or are entirely absent, and although the maxillae are so highly adapted for the tasting of sweets that it is difficult to recognise in them the parts usually found in the maxilla of mandibulate Insects.

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_Eriocephala_ in both these respects connects the Lepidoptera with Mandibulata: the mandibles have been shown by Walter[340] to be fairly well developed; and the maxillae are not developed into a proboscis, but have each two separate, differentiated—not elongated—lobes, and an elongate, five-jointed, very flexible palpus. The moths feed on pollen, and use their maxillae for the purpose, somewhat in the style we have mentioned in Prodoxidae. The wings have no frenulum, neither have they any shoulder, and they probably function as separate organs instead of as a united pair on each side: the modification of the anterior parts of the hind wing—whereby this wing is reduced as a flying agent to the condition of a subordinate to the front wing—does not here exist: the hind wing differs little from the front wing in consequence of the parts in front of the cell being well developed. There is a small jugum. These characters have led Packard to suggest that the Eriocephalidae should be separated from all other Lepidoptera to form a distinct sub-Order, Lepidoptera Laciniata.[341] The wing-characters of _Eriocephala_ are repeated—as to their main features—in Hepialidae and Micropterygidae; but both these groups differ from _Eriocephala_ as to the structure of the mouth-parts, and in their metamorphoses. Although _Eriocephala calthella_ is one of our most abundant moths, occurring in the spring nearly everywhere, and being easily found on account of its habit of sitting in buttercup-flowers, yet its metamorphoses were till recently completely unknown. Dr. Chapman has, however, been able to give us some information as to the habits and structure of the larvae, in both of which points the creature is most interesting. The eggs and young larvae are "quite {435}unlike our ideas of a Lepidopterous Insect;" the former have a snowy or mealy appearance, owing to a close coating of minute rods standing vertically on the surface of the egg, and often tipped with a small bulb. The larva lives amongst wet moss and feeds on the growing parts thereof; it is not very similar to any other Lepidopterous larva: Dr. Chapman suggests a similarity to the Slug-worms (Limacodids), but Dyar is probably correct in thinking the resemblances between the two are unimportant: the larva of _Eriocephala_ possesses three pairs of thoracic legs, and eight pairs of abdominal appendages, placed on the segments immediately following the thorax; on the under-surface of the ninth and tenth abdominal segments there is a sucker, trifoliate in form; this is probably really situate entirely on the tenth segment: the body bears rows of ball-appendages, and the integument is beautifully sculptured. The head is retractile and the antennae are longer than is usual in caterpillars. This larva is profoundly different from other Lepidopterous larvae inasmuch as the abdominal feet, or appendages, are placed on different segments to what is customary, and are of a different form. Unfortunately the pupa has not been procured, but there is some reason for supposing that it will prove to be more like that of Tineidae than like that of Micropterygidae.

The New Zealand genus _Palaeomicra_ is only imperfectly known. Meyrick considers it the "most ancient" Lepidopteron yet discovered; and it would appear that its relations are with _Eriocephala_ rather than with _Micropteryx_. From information he has kindly given to us, we are able to say that this moth possesses mandibles but no proboscis.

FAM. 47. MICROPTERYGIDAE.—Small moths of metallic colours, without mandibles, with elongate maxillary palpi: without frenulum: both wings with a complex system of wing-veins: on the hind wings the area anterior to the cell is large, and traversed by three or four elongate, parallel veins.

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There are no mandibles, but there is a short, imperfect proboscis. Larva (Fig. 210) without any legs, mining in leaves. The pupa (Fig. 211) is not a pupa obtecta, but has the head and appendages free, and is provided with enormous mandibles. Although these Insects in general appearance resemble _Eriocephala_ to such an extent that both have been placed in one genus, viz. _Micropteryx_, yet the two forms are radically distinct. The most remarkable point in _Micropteryx_ is the metamorphosis; the female moth is furnished with a cutting ovipositor, by the aid of which she deposits an egg between the two layers of a leaf after the manner of a saw-fly;[342] the larva mines the newly-opened leaves in the early spring, and feeds up with rapidity; it by some means reaches the ground, and there pupates in a firm but thin cocoon, with grains of earth fastened to it; in this it passes the greater part of its life as a larva, changing to a pupa very early in the following spring. The pupa is unlike any other Lepidopterous pupa, but is similar to those of Trichoptera; neither the head nor the appendages are glued to the body or to one another, but are free, so that the pupa can use the appendages to a considerable extent; it is furnished with enormous mandibles (Fig. 211, C, D), which are detached and shed after emergence.[343] In the interval between {437}the larval period of feeding and the imaginal instar, the phenomena of life are essentially like those of Trichoptera. The larva has not been at all satisfactorily studied; the spiracles appear to be excessively minute, but have been ascertained by Dr. Chapman to be normal in number and position.

All the information we possess points to profound distinctions between _Micropteryx_ and _Eriocephala_, for whereas in the former the mandibles drop off from the pupa, so that the imago has no mandibles, in the latter the mandibles exist, as they do in several other true Lepidoptera. As the history of the mandibles is not known in other Lepidoptera (where they are present in the larva but wanting in the imago), it is premature to conclude that no other Lepidoptera suffer the actual loss of the mandibles as _Micropteryx_ does, though there is nothing to lead us to believe that in any other Lepidopterous pupa are the mandibles specially developed as they are in _Micropteryx_. This pupa is in fact quite unique in this Order of Insects. When the history of the pupal mandibles is known, we shall be able to decide whether they are secondary structures, like the deciduous, supplementary mandibles found in Otiorhynchides (Coleoptera, Rhynchophora).

{438}CHAPTER VII

DIPTERA—OR FLIES; APHANIPTERA—OR FLEAS; THYSANOPTERA—OR THRIPS

ORDER VII. DIPTERA

_Wings two, membranous, usually transparent and never very large; behind
the wings a pair of small erect capitate bodies—halteres—frequently
concealed under membranous hoods. No distinct prothorax, all the
divisions of the thorax being united to form a large mass. Mouth-parts
very variable, formed for suction not for biting, frequently assuming the
form of a proboscis that can be retracted and concealed in a cleft of the
under side of the head. The metamorphosis is very great, the larvae
bearing no resemblance whatever to the perfect Insects, but being usually
footless grubs or maggots; frequently the head is indistinct, small, and
retracted. Pupa variable, either exposed and rather hard, with the
appendages of the body more or less adherent; or enclosed in a scaly
capsule looking like a seed, and when extracted, soft and delicate, with
the appendages not fastened to the body incapable of movement._

This definition of the Diptera, or two-winged flies, is framed without reference to the fleas, which are wingless, or to a few other parasitic wingless Diptera, such as the sheep-tick. Although the Order is of enormous extent, these exceptional cases are remarkably few. About 40,000 species of Diptera have been discovered, but these are only a tithe of what are still unknown to science. The Order is not a favourite one with entomologists, and by the rest of the world it may be said to be detested. Flies do not display the sort of intelligence we appreciate, {439}or the kind of beauty we admire, and as a few of the creatures somewhat annoy us, the whole Order is only too frequently included in the category of nuisances that we must submit to. Moreover, the scavenger-habits that are revealed, when we begin to study their lives, are very repugnant to many persons. It is therefore no wonder that flies are not popular, and that few are willing to study them, or to collect them for observation. Nevertheless, Diptera have considerable claims to be classed as actually the highest of Insects physiologically, for it is certainly in them that the processes of a complete life-history are carried on with the greatest rapidity and that the phenomena of metamorphosis have been most perfected. A maggot, hatching from an egg, is able to grow with such rapidity that the work of its life in this respect is completed in a few days; then forming an impenetrable skin it dissolves itself almost completely; solidifying subsequently to a sort of jelly, it in a few days reconstructs itself as a being of totally different appearance and habits, in all its structures so profoundly changed from what it was that the resources of science are severely taxed to demonstrate any identity of the organs of the two instars.

A good study of the comparative anatomy of Diptera has never been made; Baron Osten Sacken, one of our most accomplished Dipterologists, has recently stated that "the external characters of the Diptera have as yet been very insufficiently studied." We shall therefore only trouble the student with a few observations on points of structure that are of special importance, or that he will find frequently alluded to. The head is remarkable {440}for its mobility, and is connected with the thorax by a slender concealed neck that permits the head to undergo semi-rotation. A large part—sometimes nearly the whole—of the exposed surface of the head is occupied by the faceted eyes. It is usually the case that the eyes are larger in the male than in the female, and the sexual discrepancy in this respect may be very great. When the eyes of the two sides meet in a coadapted line of union the Insect is said to be "holoptic," and when the eyes are well separated "dichoptic."[344] The holoptic condition is specially characteristic of the male, but in some forms occurs in both sexes. There is no definite distinction between holoptic and dichoptic eyes. The eyes may be enormous, Fig. 238, without actually uniting, and in the cases where actual contiguity occurs, it takes place in different manners.[345] The eyes are frequently during life of brilliant colours and variegate with stripes or spots; this condition disappears speedily after death, and it is uncertain what the use of this coloration may be.[346] The eyes are frequently densely set with hairs between the almost innumerable facets. These facets frequently differ in size according to their position in the organ. The curious double eye of the male _Bibio_ (cf. Fig. 224) is well worth notice. There are usually three small ocelli placed very near together on the middle of the summit of the head.

The antennae are of considerable importance, as they offer one of the readiest means of classification. The families placed by systematists at the commencement of the Order have antennae similar to those of the majority of Insects, inasmuch as they consist of a series of segments approximately similar to one another, and arranged in a linear manner (Fig. 213, A). The number of these joints is never very great, but reaches sixteen in certain Tipulidae, and falls as low as eight in some Bibionidae. In certain cases where the antennae of the male are densely feathered (_Chironomus_, e.g.), the number of joints is in that sex greatly augmented, but they are imperfectly separated. This form of antenna gives the name Nemocera to the first series of Diptera.

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The majority of flies have antennae of another form, peculiar to the Order, viz. three segments, the outer one of which is of diverse form, according to the genus or species, and bears on its front a fine projecting bristle, frequently feathered, as in Fig. 213, F; and often distinctly divided into two or more joints. This form of antenna is found in the series Aschiza and Schizophora; it is well exemplified in the common house-fly, where the organs in question hang from the forehead, and are placed in a hollow formed for their reception on the front of the head. Flies with this form of antennae are called Athericerous. Between the two forms of antennae we have mentioned there exists what may, speaking roughly, be called an intermediate condition, or rather a variety of intermediate conditions, associated in the series Brachycera (Fig. 213, B to D).[347] Here there are three (sometimes one or two) segments and a terminal appendage, but the appendage is usually compound (often so distinctly compound that it is evidently a series of partially, or even completely, separate joints, Fig. 213, B): the appendage in these cases is terminal, that is to say it is placed, not as in the Eumyiidae on the front of the joint that bears it, but (in the great majority of Brachycera) at the tip thereof; this appendage is often conical and pointed, often hair-like. Exceptional forms of antenna are found in the parasitic flies of the series Pupipara. In the Order generally the two basal joints of the antennae are evidently distinct in function from the others, and form the "scape"; the {442}part of the antenna beyond the scape is called the "flagellum"; an appendage of the flagellum is called "arista" when bristle-like, when thicker "style." In the basal joint of the antenna there is a complex nervous structure known as Johnston's organ. It is specially well developed in _Culex_ and _Chironomus_, and is larger in the male than it is in the female. Child has found something of the kind present in all the Diptera he has examined, and he considers that an analogous structure exists in Insects of other Orders. He thinks it is concerned with the perception of vibration, there being no sharp distinction between auditory and tactile sensation.[348]

About one-half of the Diptera possess a peculiar structure in the form of a head-vesicle called "ptilinum." In the fly emerging from the pupa this appears as a bladder-like expansion of the front of the head; being susceptible of great distension, it is useful in rupturing the hard shell in which the creature is then enclosed. In the mature fly the ptilinum is completely introverted, and can be found only by dissection; a little space, the "lunula," just under an arched suture, extending over the point of insertion of the antennae remains, however, and offers evidence of the existence of the ptilinum. This structure is also of importance in classification, though, unfortunately, it is difficult to verify.[349]

No point of Insect morphology has given rise to more difference of opinion than the mouth of Diptera; and the subject is still very far from being completely understood. The anatomy and morphology of the mandibulate Insect-mouth are comparatively simple (though not without greater difficulties than are usually appreciated); and it has been the desire of morphologists to homologise the sucking mouth of Diptera with the biting mouth; hence the view that the appendages of three segments are separate and distinct in the fly's mouth is taken for granted, and it is further assumed that some of the secondary parts of the appendages of the biting mouth can also be recognised in the sucking mouth. The anatomy of the mouth-parts is, however, {443}subject to great diversity of structure within the limits of the Order itself, even the two sexes in some species differing profoundly in this respect.[350] In the majority of the family Oestridae the mouth-parts are practically absent, and no definite entry to the alimentary canal can be perceived (Fig. 245). Besides this condition and its antithesis (Fig. 214), the complex assemblage of lancets seen in the Breeze-flies that draw blood, there is a great variety of other anatomical conditions.

Although, as we have said, great diversity of opinion exists, yet on the whole the majority of Dipterologists accept a view something to the following effect:—the labrum, or the labrum combined with the epipharynx, is frequently much prolonged; the tongue—hypopharynx—may also be much prolonged, and may form, in apposition with the labrum, a more or less imperfect tube for ingestion of the nutriment; the labium is more or less membranous or fleshy, and acts as a sheathing organ, its tips—called labella—-being in some cases developed to a quite extraordinary extent. As to the other parts of the mouth there is less agreement; the pointed organs (Fig. 214, A, _b_ _b_) are by {444}many identified as mandibles, while another pair of pointed processes (_c_ _c_) are considered to be parts of a maxilla, and the palpi (_f_ _f_) are by some considered to be maxillary palps. The Danish entomologist, Meinert, has published the best anatomical description of many of the diverse kinds of Dipterous mouth.[351] He, however, takes a different view of the morphology; he considers that not only may parts of the appendages of the mouth be much modified during the early stages of the individual development, but that they may be differently combined, even parts of the appendages of two segments being brought together in intimate combination. He has also pointed out that the mandibulate and sucking mouth are mechanical implements constructed on opposed principles; the main object of a biting mouth being the fixing and perfecting of the articulations of the mouth, so that great power of holding may be attained with a limited but definite power of movement. In the sucking mouth the parts are intimately associated for simple protrusion. Hence the two kinds of mouth must have been distinguished very early in the phylogeny, so that we must not expect to find a great correspondence between the parts of biting and sucking mouths. He apparently also considers that not only the appendages of a head-segment, but also part of the body of the segment, may be used in the construction of the mouth-organs. Meinert's views allow a much greater latitude of interpretation of the parts of the Dipterous mouth; had he contented himself with enunciating them in the manner we have followed him in summarily describing, they would have been recognised as a formidable obstacle to the facile adoption of the ordinary views. He has, however, accompanied his general statement with a particular interpretation and a distinct nomenclature, neither of which is it possible to adopt at present, as they have no more justification than the ordinary view. So that instead of one set of doubtful interpretations we have two.[352] In so difficult a question as homologising the trophi of different Orders of Insects we ought to use {445}exhaustively every method of inquiry: and from this point of view the development is of great importance. This has, however, as yet thrown but little light on the subject, this study being a very difficult one owing to the profound changes that take place during metamorphosis, the diversity of the parts in the early stages of Diptera, and the possibility that the larval conditions may themselves have been greatly changed in the course of the phylogeny. Miall informs us, however, that in _Chironomus_ as well as in _Corethra_ the new parts of the mouth of the imago are developed within those of the larva.[353] This may permit of an identification of the main divisions of the mouth, at any rate in these cases. Lowne has to some extent traced the development in the blowfly, and he does not agree with the usual interpretation of the parts in the adult.

The mouth is of considerable importance in the classification of Diptera. The Nemocera are remarkable from the linear development and flexibility of the palpi, which are nearly always at least three- or four-jointed; this condition occurring in no other Diptera. The palpi attain an extraordinary development in some Culicidae; in the genus _Megarrhina_ they are nearly as long as the body, and project in front of the head after the fashion of the palpi of Lepidoptera. In the Brachycera the sclerites or hard parts of the mouth reach a maximum of development, and in Tabanidae (Fig. 214), Nemestrinidae and Bombyliidae are often quite disproportionate to the size of the Insect. In many of the Eumyiid flies the soft parts are greatly developed, and capable of a variety of movement, the proboscis as a whole being protrusible, and having an elbow-joint in the middle.

The thorax is remarkable from the absence of distinct separation into the three divisions that may usually be so easily distinguished in Insects. The perfect combination of the three segments adds much to the difficulty of arriving at general conclusions as to the identification of the parts; hence considerable difference of opinion still prevails. It was formerly supposed that a segment from the abdomen was added to the thorax of Diptera as it is in Hymenoptera, but this has been shown by Brauer to be erroneous. Indeed, according to Lowne, the abdominal cavity is increased by the addition of the small posterior area of the thorax; it being the mesophragma that separates the {446}second and third great divisions of the body-cavity. The prothorax is always small, except in a few of the abnormal wingless forms (_Melophagus_); in _Nycteribia_ (Fig. 248) the mesothorax forms the anterior part of the body; the head and such parts of the prothorax as may be subsequently discovered to exist being placed entirely on the dorsum of the body. The mesothorax in all the winged Diptera forms by far the larger portion of the thoracic mass, the prominent part of it, that projects backwards to a greater or less extent over the base of the abdomen, being the scutellum. The first or prothoracic stigma is remarkably large and distinct, and is by some called mesothoracic. Another large stigma is placed very near to the halter (or balancer); the metathorax being very small. An imperfect stigma is said by Lowne to exist in the blowfly near the base of the wing. The number of abdominal segments externally visible is very diverse; there may be as many as nine (in the male _Tipula_), or as few as five, or even four, when the basal segment is much concealed; the diminution is due to certain segments at the extremity being indrawn and serving as a sort of tubular ovipositor in the female, or curled under the body and altered in form in the other sex, so as to constitute what is called a "hypopygium." In the female of Tipulidae the body is terminated by some horny pieces forming an external ovipositor. In nearly all Diptera the feet are five-jointed; the claws are well developed, there being placed under each of them a free pad or membrane, the "pulvillus"; there may be also a median structure between each pair of claws, of diverse form, the "empodium."

On the surface of the body of many flies there will be seen an armature of pointed bristles; these flies are called "chaetophorous"; where no regularly arranged system of such bristles exists the fly is "eremochaetous." In some families the arrangement of these bristles is of importance in classification, and a system of description has been drawn up by Baron Osten Sacken: this branch of descriptive entomology is known as chaetotaxy.[354]

The wings are of great importance in classifying Diptera; but unfortunately, like the other parts, they have not received an exhaustive anatomical study, and Dipterologists are not agreed as to the names that should be applied to their parts.

{447}

We give below figures of two systems that have been used by eminent Dipterologists for the description of the nervures and cells. The comprehension of these features of the Dipterous wing will be facilitated by noticing that the wing—being extended at right angles to the body—is divided by the longitudinal nervures into two great fields, anterior and posterior, with an interval between them: this interval is traversed only by a short cross-vein (marked x in Fig. 215 A, and i in B). This cross-vein may be placed near the base or nearer to the tip of the wing; it is of importance because no nervure in front of the median area traversed by it can correspond with a nervure placed behind it in another wing. The very different nature of the nervuration in the two wings we have figured will readily be appreciated by an inspection of the parts posterior to the little cross-vein. On the hind margin of the wing, near the base, there is often a more or less free lobe (Fig. 215, B, O) called the "alula": still nearer to the base, or placed on the side of the body, may be seen one or two other lobes, of which the one nearer the alula is called the "tegula," or (when a lobe behind it is also present) the "upper tegula," (the "antitegula" of Osten Sacken); the other being the "lower tegula." These two terms are erroneous, the word tegula being definitely applied to another part of the Insect-body. In speaking of this structure in the following pages, we have preferred to call it the {448}"squama."[355] Those Muscidae in which the squama covers the halter like a hood are called "calypterate." In Fig. 216, we represent these structures, and in the explanation have mentioned the synonyms. The terms we think most applicable to the three lobes are alula, antisquama, squama. The squama may be called "calypter" when it covers the halter.

The halteres—commonly called balancers or poisers—are perhaps the most characteristic of all the Dipterous structures, though they are absent in most of the few wingless forms of the Order. Outside the Diptera similar organs appear to exist only in male Coccidae. The pair of halteres is placed on the metathorax, one on each of the pleural regions. They are believed to be the homologues of the hind wings; Weinland states[356] that certain canals existing in the interior of the halter correspond to wing-nervures. The halter may be described as a small rod-like body with a head like a pin, this terminal part being, however, rather variable in form. We have already stated that in many Diptera the squama forms a hood, the position of which leads to the belief that it is an important adjunct to the halter. Although the exact functions of the halteres are far from clear, it is certain that they are highly complex bodies, of extremely delicate structure: they are doubtless sense-organs, possessing as they do, groups of papillae on the exterior and a chordotonal organ (a structure for assisting the perception of sound) in the basal part; each halter is provided with four muscles at the base, and can, like the wings, execute most rapid vibrations. Seeing that they are the homologues of wings, it is a remarkable fact that in no Diptera are they replaced by wings, or by structures intermediate between these two kinds of organs.

INTERNAL STRUCTURE.—Information about the internal anatomy {449}is by no means extensive. The tracheal system is highly developed, and has air-sacs connected with it; a large pair at the base of the abdomen being called aërostats by Dufour. Inside the thoracic spiracles there are peculiar structures supposed by some to be voice-organs, while the abdominal spiracles are said to be remarkably simple in structure. Lowne says that there are ten or eleven pairs of spiracles in the Blow-fly; one of these, near the base of the wing, is peculiar in structure, and may not be a true stigma; he calls it a tympanic spiracle; it seems doubtful whether there are more than seven abdominal pairs. The alimentary canal is very elongate, and is provided with a diverticulum, the crop; this is usually called the sucking stomach, though its function is extremely doubtful. The Malpighian tubes are four in number, and are very elongate; in several groups of Nemocera there are, however, five Malpighian tubes, a number known to occur in only very few other Insects. The nervous system is remarkable on account of the concentration of ganglia in the thorax, so as to form a thoracic, in addition to the usual cephalic, brain. For particulars as to the positions of the ganglia and the great changes that occur in the lifetime, the student should refer to Brandt, to Künckel, and to Brauer.[357] Much information as to the internal anatomy of the Blowfly is given by Lowne, but it is doubtful to what extent it is applicable to Diptera in general.[358]

The LARVAE of Diptera are—so far as the unaided eye is concerned—without exception destitute of any kind of adornment, the vast majority of them being of the kind known as maggots. None of them have true thoracic legs; though in the earlier groups, pseudopods or protuberances of the body that serve as aids in locomotion are common. Unlike what occurs in other Orders the arrangement of these pseudopods on the body differs greatly in various forms; in a few cases they are surmounted by {450}curved hairs. The most important distinction in external form in Dipterous larvae is that while those that are thorough maggots possess no visible head, others have a well-marked one (Fig. 225); these are therefore called "eucephalous": they have a mouth of the mandibulate type. In some other Dipterous larvae the head is more or less reduced in size, and in the acephalous forms there is only a framework of a few chitinous rods to represent it. The nervous system in the most completely headless larvae is very remarkable, all the ganglia being concentrated in a single mass placed in the thorax. The tracheal system exhibits a great variety; some larvae have stigmata arranged along the sides of the body after the fashion normal in Insect-larvae; these are called "peripneustic"; as many as ten pairs of stigmata may be present in these cases, but nine pairs is much more common. Other larvae have a pair of stigmata placed at the termination of the body, and another pair near the anterior extremity, the two pairs communicating by large tracheal trunks extending the length of the body; these larvae are said to be "amphipneustic": this is the condition usual in the more completely acephalous larvae. Others have only the terminal pair of spiracles, and are styled "metapneustic." Some begin life in the metapneustic state and afterwards become amphipneustic. In the aquatic larva of _Corethra_ there are no spiracles, though there is an imperfect tracheal system. Many Dipterous larvae that live in water or in conditions that prevent access of air to the body have remarkable arrangements for keeping the tip of the body in communication with the atmosphere. The stigmata in metapneustic and amphipneustic larvae are very remarkable compound structures, exhibiting however great diversity; their peculiarities and uses are not well understood; it appears very doubtful whether some of them have any external opening. Reference may be made, as to the variety of structure, to Meijere's paper[359] from which we take the accompanying figure of a posterior stigmatic apparatus in _Lipara lucens_. It appears that there is a compound chamber—"Filzkammer"—terminating externally in lobes or fingers—"Knospen" and appearing as marks on the outer surface: this chamber is seated on a tracheal tube, and is, Meijere thinks, probably a secondary growth of the trachea coming to the outer surface. It is traversed by what may be {451}considered the original tracheal tube, opening externally as an external stigmatic scar—"Stigmennarbe"—and with a second or inner scar placed internally. We may conclude from what is already known that these structures will be found to differ in the same larva according to the stage of its development.

An extremely valuable summary of the characters and variety of Dipterous larvae has been given by Brauer,[360] from which it appears that the larvae of the first half of the family exhibit great variety and have been much studied, while the more purely maggot-like forms of the Muscidae have, with one or two exceptions, been little investigated.

The PUPAL instar is of two distinct kinds. First, we meet with a pupa like that of Lepidoptera, viz. a mummy-like object, or pupa obtecta, in which there is a crisp outer shell, formed in part by the adherent cases of the appendages of the future imago. This condition, with a few exceptions to be subsequently noticed, obtains in the Nemocera and Brachycera. It is exhibited in various degrees of perfection, being most complete in Tipulidae; in other forms the shell is softer and the appendages more protuberant. The second kind of pupa is found in the Cyclorrhaphous flies; it has externally no marks except some faint circular rings and, frequently, a pair of projections from near one extremity of the body; occasionally there is a single prominence at the other extremity of the body. This condition is due to the fact that the larva does not escape from the skin at the last ecdysis, but merely shrinks within it, so that the larval skin, itself contracted and altered by an excretion of chitin, remains and forms a perfect protection to the included organism. This kind of pupa looks like a seed, and is well exemplified by the common Blow-fly. The capacity for entering on such a condition is evidently correlative with the absence of a larval head. The metamorphosis in this curious little barrel goes on in a different manner to what it does in the pupa {452}obtecta. A good name for the whole structure of this instar has not been found. Older authors called it "pupa coarctata," or "nympha inclusa"; Brauer speaks of it as a "compound pupa"; ordinarily in our language it is called a "puparium," a term which is more applicable to the case alone.

In species having a pupa obtecta the larval skin is cast after the chief processes of the external metamorphosis have occurred, and then an exudation of chitin hardens the general surface. In the "compound pupa" of the Blow-fly there is for a considerable period no formed pupa at all, but merely a shell or case containing the results of histolysis and the centres for regeneration of new organs; the chitin-exudation to the exterior of the larval skin occurs in the early part of the series of metamorphic changes, and the organism breaks down to a cream within the shell thus formed, and then gradually assumes therein the condition of a soft, nymphoid pupa. The exceptional conditions previously referred to as exhibited by a few forms are certain cases in which a more or less perfect pupa obtecta is found within the last larval skin, as is the case in _Stratiomys_. Another highly remarkable condition exists in the Hessian fly, and a few other Cecidomyiids, where the Insect apparently makes an exudation which it uses as a covering case, independent of the larval skin; this latter being subsequently shed inside the case, so that this condition of coarctate pupa differs from that we have described as existing in Cyclorrhaphous flies, although the two are superficially similar. In the Pupipara the larval stage is passed in the body of the mother, which produces a succession of young, nourished one at a time by the secretion of glands; this young is born as a full-grown larva that becomes at once a pupa.

METAMORPHOSIS.—As it is in Diptera that the phenomena of Insect-metamorphosis have reached their highest development we endeavoured to give some idea of their nature in the previous volume, therefore we need give only a brief sketch of the chief features of Dipterous metamorphosis. The Blow-fly undergoes a rapid embryonic development, the later stages of which are, on the whole, of a retrogressive nature. On the emergence of the young maggot it feeds up rapidly, the rapidity varying greatly according to circumstances, and then when full-grown rests. While resting, a process of internal liquefaction, called histolysis, is going on, and the maggot contracts and exudes an excretion {453}that hardens its skin. At the time this hard skin has become complete, or soon after, the maggot inside has dissolved into a cream contained in a sac inside the shell; this cream becomes reconstituted into a fly by a gradual process of growth and development of certain minute portions of the body—the imaginal discs or folds, the histoblasts and neuroblasts that were exempt from the histolytic process: in the early stages of the reconstitution the general structure is, of course, altogether vague, and this condition—purely one of transition—is called the pronymph; the nymph becomes gradually developed: it corresponds vaguely with the pupa obtecta of the early groups of Diptera, but is soft like the pupa of Hymenoptera. This nymph gradually develops into the fly itself, the external parts being first completed and the internal organs elaborated subsequently. The sexual organs do not undergo metamorphosis like other internal organs, there being a gradual (though irregular or interrupted) growth of them in the young larva, till they are completed some time after the emergence of the perfect fly. The processes in the Blow-fly have been studied by numerous able histologists of various nationalities, and have recently been described by Lowne in our own language.[361] Comparatively little has been done in studying the corresponding phenomena in other Diptera. Weismann has investigated the development of _Corethra_, and Miall that of _Chironomus_. These two flies belong to a division of Diptera different from that which includes the Blow-fly, and they display a condition of the metamorphic processes allied to what occurs in Lepidoptera, as well as to that which takes place in the Blow-fly. Imaginal folds are formed, but they only appear much later in the life, and they are much less distant from the positions they will, when developed, occupy in the imago. In _Chironomus_, according to Miall, the imaginal folds only appear in the last larval instar, but they grow with such rapidity that the legs and wings of the future fly can be distinguished in the larva, even before pupation; thus when the activity of the larva ceases but little change is required to complete the obtected pupa. In the Blow-fly some of the imaginal folds have been {454}traced back to the embryo; how many centres for the new growth there may be is uncertain, for though there are upwards of sixty for the outer body, the number of regenerative centres for the internal organs is not ascertained. The peculiar central nervous mass, mentioned in our remarks on the larva, consists of two kinds of tissue mixed together in a complex manner; one of these kinds is functionally active during the larval life and at the metamorphosis undergoes histolysis, while the other, or embryonic, portion develops into the nervous system of the fly.