Fig. 212—A Dipteron (Fam. Syrphidae), Cheilosia chrysocoma. Britain. A, Adult larva; B, the pupa; C, nymph, extracted from pupa; D, imago. (From Weyenbergh.)
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 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.
Fig. 213—Antennae of flies. A, The two antennae of Glaphyroptera picta (Mycetophilidae); B, antenna of Hexatoma pellucens (Tabanidae); C, of Asilus crabroniformis (Asilidae); D, of Leptis scolopacea (Leptidae); E, of Dolichopus undulatus (Dolichopidae); F, of Volucella bombylans (Syrphidae). (After Wandolleck.)
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 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, 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.
Fig. 214.—Mouth-parts of the common blood-sucking fly, Haematopota pluvialis ♀. A, Viewed from beneath, the proboscis removed; a, labrum; b, b, cultelli (mandibles of other anatomists); c, c, scalpella (maxillae of other anatomists); d, part of ventral scutum of second metamere; e, e, f, f, parts of palpi; g, hypopharynx and pellucid salivary duct; h, salivary receptacle; i, salivary duct; k, membranous part of second metamere; l, pharynx: B, labrum, pharynx, hypopharynx, separated, seen from beneath; a, labrum; b, hypopharynx; c, salivary duct; d, pharynx; e, protractor muscles: C, proboscis (labium) from beneath; a, scutum proboscidis; c, c, labella; d, d, retractor muscles. (After Meinert).