Immature Stages of Diptera
Clausen (1940) remarked that a noticeable degree of specialization has taken place among the immature stages of parasitic groups of Diptera as compared with the general predators, and no attempt will be made here to group the latter with respect to egg, larval, and pupal types. For an extended comparative study of the mature larvae and pupae of a considerable number of families, both orthorrhaphous and cyclorrhaphous, an early but excellent reference is De Meijere (1917).
Clausen (1940) discusses the different stages of Diptera as follows:
The Egg.--The egg types are:
1. The muscoidiform is the most common; it is distinguished by having a thin and delicate chorion, largely lacking in ornamentation, and ranges in form from elliptical to elongate cylindrical. In the Tachinidae and Sarcophagidae, these eggs usually undergo complete or partial uterine incubation. Relatively few are deposited internally in the host. Other parasitic and specialized predaceous groups having membranous eggs are the Agromyzidae (Cryptochaetum), Phoridae, Pyrgotidae, Bombyliidae, Cecidomyiidae, and Nemestrinidae.
2. The encrusted egg is similar in form to the above but has a relatively heavy waxy incrustation, which may bear various striate or reticulate markings over the surface of the chorion, giving it a pearly‑white color. This type is found generally in the Syrphidae, Drosophilidae, and Ochthiphilidae.
3. The pedicellate egg is ellipsoidal to elongate cylindrical in form, with a thin and transparent chorion, and bears a stalk at one end by means of which it is attached to the integument of the host or to an internal organ. In the Tachinidae (genus Carcelia only), the stalk is at the anterior end and the tip is expanded to form an adhesive structure, while in the Conopidae it is similarly situated and bears the micropyle at its tip. The egg of the parasitic cecidomyiid, Endopsylla endogena Kieff., has a minute stalk that is embedded in the wing of the psyllid host. Partial or complete uterine incubation takes place in eggs of Carcelia.
4. The macrotype egg is found only in the Tachinidae; it is broadly oval to ellipsoidal in outline, with a thick, tough chorion on the dorsum and sides, and the ventral surface is fiat and membranous. This type of egg is attached externally to the integument of the host by its flat ventral surface. Strong adhesion is ensured by a mucilaginous material, which accompanies the egg at deposition. Very few of these eggs show any evidence of uterine incubation.
5. The microtype egg is, as its name implies of minute size; it may be ovate or almost circular in outline, with the ventral surface thin and membranous, for attachment to a leaf or other surface, and the dorsum and sides heavy, tough, and usually highly sclerotized, with various surface markings. Many genera and species of Tachinidae deposit this type of egg. The eggs of the Cyrtidae, which are of somewhat different form and adhere to the substratum by the posterior end rather than the ventral side, show no embryonic development at the time of deposition, have a long period of incubation, and hatch without the intervention of the host. In the great majority of tachinid species, uterine incubation is complete, but hatching takes place only after ingestion of the eggs by the host. This type corresponds to the microtype egg of the Trigonalidae in the Hymenoptera.
First‑instar Larvae.--The first‑instar larvae of the parasitic and restricted predaceous groups show a degree of specialization comparable with that of the egg forms. Only the forms that depart from the normal for the higher groups to which they belong will be considered here. The greatest variety in larval forms is found among the Tachinidae, which contains four of the five types listed below.
1. The muscoidiform larva is the generalized type of the superfamily and includes the less specialized Tachinidae and Sarcophagidae, the Pyrgotidae, etc. It is a plain maggot, without adaptive characters.
2. The microtype larva is that which originates from the microtype egg of many Tachinidae. It is distinguished by its small size, lack of extensive cuticular armature and the reduction and simplification of the mouthparts. It hatches from the microtype egg in the digestive tract of the host.
3. The planidium larva is found generally among the Cyrtidae, Bombyliidae and Nemestrinidae and frequently in the Tachinidae and in a few species of Sarcophagidae. A rather elongated form and a heavy integument, often highly sclerotized, frequently bearing heavy spines, scales or plates, and long caudal setae, except among the majority of tachinid species, distinguish it. These adaptations provide for an active and relatively long free life before the host is reached. All larvae of this type are followed by a generalized second instar which lacks the above adaptations and is capable of very little ordered movement. [For a discussion of the planidium larva in the different orders, see Clausen (1940), pages 17‑19].
4. The vesiculate type of larva is much less common in the parasitic Diptera than in the Hymenoptera. Among the Tachinidae, it is found in Plagia trepida Meig., and the vesicle is in the form of a large plate occupying the main portion of the ventral face of the last segment. It occurs also in the Pipunculidae and possibly in the Conopidae, though in the latter family the first instar has been little studied but the vesicle has thus far been noted in the following instars.
5. The caudate larva is readily recognized by its paired caudal appendages, which persist in the following instars. Among the entomophagous Diptera, it is found in the agromyzid genus Cryptolaemus that is parasitic internally in monophlebine Coccidae. This larval form is found in some aquatic or semiaquatic species that are not entomophagous in habit, such as the calliphorid, Wilhelmina nepenthicola Vill., of Borneo.
Mature Larvae.--Although there is a marked convergence in form in the mature larvae as compared with the specialized first instar, yet many groups have characters that can be readily recognized. We are particularly concerned with the families or lower groups having parasitic larvae and with the highly specialized predators. The general characters by means of which they may be recognized, or at least narrowed to certain limits, are as follows:
Cecidomyiidae.--The head feebly developed and lacking mandibles; body color often red, orange, or yellow; 13 body segments with lateral abdominal spiracles present; of small size.
Cyrtidae.--Body much the widest in the abdominal region, tapering markedly cephalad; a constriction between thorax and abdomen and segmentation evident only anteriorly; anterior and posterior spiracles present, the latter simple in form.
Bombyliidae.--Body crescentic, tapering at both ends, abdomen 9-segmented, with integument glistening and bare; anterior and posterior spiracles present, the latter on the penultimate segment and comprising 8-12 openings arranged in a crescent or semicircle around a distinct button; the anterior pair similar but smaller, or fan-shaped.
Phoridae.--Segmentation distinct, though obscured by supplementary folds, the parasitic species usually lacking the fleshy integument processes; the buccopharyngeal armature in 3 parts with the mandibular sclerite unpaired; anterior and
posterior spiracles present, slightly elevated, the latter usually with 4 openings.
Pipunculidae.--Segmentation indistinct and mouthparts reduced; anterior and posterior spiracles present, somewhat elevated, the latter pair situated at the lateral margins of a single heavily pigmented peristigmatic plate, and each usually having 3 openings; some species with the posterior spiracles situated in a pronounced depression.
Conopidae.--Body markedly pear‑shaped, with the thoracic segments much attenuated, and the integument clothed with minute setae or tubercles; anterior spiracles often lacking, but when present fan‑shaped, with many openings; posterior spiracles very large, convex to hemispherical and heavily sclerotized; the minute
circular openings arranged in rows or groups, totaling 400 to 700, about a large button; a sclerotized process surmounted by several blunt spines occurs slightly dorsad of each posterior spiracle; paired anal vesicles present in some species.
Pyrgotidae.--Body pear‑shaped, with the integument bare and glistening; anterior spiracles stalked and fan‑shaped, whereas the posterior ones are very large, of 3 main lobes with many small openings, heavily sclerotized, and usually situated at the dorsal rim of a large median depression.
Agromyzidae (genus Cryptochaetum).--Body segments distinct, each with a band of minute setae; the caudal segment with paired tubular tails which may be several times the length of the body; the anterior spiracles dart‑shaped or palmate and set in pits; the posterior spiracles in the form of heavily sclerotized, posteriorly directed hooks, with an opening at the base.
Ochthiphilidae.--Abdomen 9‑segmented, segmentation indistinct, with integument either bare or having numerous fleshy spines; anterior and posterior spiracles present, the latter consisting of three finger‑like structures terminating in simple openings that are borne upon long, cylindrical, diverging processes or stalks.
Sarcophagidae.--Body tapering markedly cephalad, with the posterior end often bluntly rounded; integument clothed with minute spines; anterior spiracles fan‑shaped, with 5 to 15 or more openings; posterior spiracles with peritreme incomplete and the slits almost parallel, lacking the button, and situated in the upper portion of a pronounced depression, the rim of which bears fleshy processes of various forms.
Tachinidae.--All general characters variable; integument usually with bands of setae on each segment, which may be complete or broken; anterior spiracles simple, plate‑like with several openings, or conical with many minute openings on the distal portion; posterior spiracles usually circular with peritreme complete, the slits most commonly numbering 3 or 4 and radiating from the spiracular button; the number of slits may range up to 30 and are most frequently straight but may be serpentine, branched or broken, or occasionally in the form of numerous small openings, irregularly placed or in rows.
The Pupa or Puparium.--There are two distinct methods of pupation in the Diptera, a fact that serves as the basis for division into two suborders. In the Orthorrhapha, the pupa emerges from the larval skin through a T‑ or + -shaped fracture, whereas in the Cyclorrhapha the larval skin becomes heavily sclerotized and hardens, forming n puparium enclosing the pupa. The lines of fracture of the puparium occur transversely on the dorsum and venter of the first abdominal segment; and a horizontal fracture, dividing the operculum into an upper and a lower half, extends across the front. The adults of all families of the suborder, except the Phoridae, Platypezidae, Pipunculidae, and Syrphidae, are provided with a ptilinum, by means of which the operculum is forced off. Greene (1925b) presented a tentative arrangement of the muscoid flies based upon the characters of the puparia.
The characters of the pupae or puparia by means of which the principal families may be recognized are as follows:
Cyrtidae.--The body robust, the head markedly ventral, with the thorax greatly arched and longer than the abdomen; prongs, hooks, and spines lacking.
Mydaidae, Asilidae, Nemestrinidae, and Bombyliidae.--The free pupa somewhat elongated in form with the head bearing several pairs of large heavily sclerotized prongs; a row of hooks on each abdominal segment, and the last segment terminating in a pair of prongs. These pupae are capable of considerable movement.
Therevidae.--As in the preceding families except that only two pairs of prongs occur on the head and one prong at the base of each wing pad.
Phoridae.--The dorsum of the puparium much less arched than the venter, with the lateral margins compressed dorsoventrally, giving it distinct boat‑like appearance; the surface dull owing to minute setae or to the persistent fleshy spines of the larva; prothoracic cornicles of the pupa extruded and small to large in size; the operculum, which includes the cornicles, may consist of a dorsal and a ventral half or of only the single dorsal plate.
Pipunculidae.--Puparium broadly oblong in outline, occasionally with the caudal spiracular area depressed; the pupal cornicles extruded and minute to very large and conical in form; the operculum in two parts, the dorsal part bearing the cornicles, or they may be situated on the line of fracture.
Conopidae.--Somewhat flattened dorsoventrally, the venter frequently more convex than the dorsum, with the segmentation indistinct and the surface smooth or wrinkled; pupal prothoracic cornicles not extruded; posterior spiracles large and elevated.
Pyrgotidae.--The venter of the anterior region much more convex than the dorsum; posterior spiracles very large and, in most species, situated on the dorsal rim of a pronounced posterior depression; surface bare and shining; pupal prothoracic cornicles not extruded.
Agromyzidae (genus Cryptochaetum).--Pointed at the anterior end, with the segmentation distinct and the paired caudal processes of the larva persistent; the anterior dart-like spiracles fully extended and terminal in position, and the caudal spiracles hooked as in the mature larva; pupal prothoracic cornicles not extruded.
Ochthiphilidae.--Oblong in form, flattened ventrally, with segmentation indistinct except in anterior region and the surface dull owing to setae or persistent fleshy spines of larva; stalked posterior spiracles as in the larva; pupal prothoracic cornicles not extruded.
Sarcophagidae.--Oblong to somewhat cylindrical in form, the segmentation indistinct and distinguished from the following family by the large posterior depression within which the spiracles are situated and the margin o# which bears fleshy processes or tubercles; spiracular characters as in the mature larva; pupal prothoracic cornicles not extruded.
Tachinidae.--Puparial characters variable, as in the mature larva, usually barrel-shaped; pupal prothoracic cornicles small and extruded in some species.