Immature Stages of Agromyzidae
Clausen (1940) commented that the egg increases considerably in size during incubation. The first instar larva is caudate, although it is often referred to in this family as embryonic, for the body is a transparent cylindrical sac 0.3-0.4 mm. long and slightly curved, with little indication of segmentation and with the caudal segment bifurcate and the lobes finger-like, with tips broadly rounded. There is no trace of a tracheal system or of heart or sensory organs. There are apparently no distinguishable mouth parts. The 2nd instar larva is a bit cylindrical and has 10 distinct segments. The tails are of markedly variable diameter, a bit shorter than the body and they terminate bluntly. Several short pointed cuticular spines occur in a transverse row on the dorsum of the 2nd abdominal segment, and similar spines completely encircle the following 5 segments, almost completely covering the 6th and 7th abdominal segments. A simple closed tracheal system is present, and the longitudinal trunks, with little evidence of branching, extend from the first thoracic to the posterior margin of the 7th abdominal segment. During the latter part of the stage, usually 6 fine branches are developed in the expanded bases of the tails. There is very little growth in this stage, and no blood flow can be detected.
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The 3rd instar larva is similar in form to that of the 2nd, the greatest difference being in the tails, which are now 1.5-2.0 times as long as the body. The bases of the tails are greatly expanded, being as wide as the preceding body segments, and the fragile filaments beyond the bulbs are of uniform diameter. The tail's hypodermis in this and preceding instars, consists of a single layer of cells with enormous nuclei. The lumen of the tail is filled with blood, although no circulation can be observed. The tracheal system is still closed, and there is a dense network of fine branches just beneath the epidermis, several of which extend into the tails for ca. 2/3rds of their length. Two transverse commissures occur in the anterior part of the body and one in the last abdominal segment (Clausen 1940/62).
The 4th larval instar is quite different from preceding instars. It is very robust and each of the 10 distinct body segments bears a band of minute setae. The tails are exceedingly long, being 3-4 times the body length. Except for the basal bulb, the tails are very slender, kinked and irregular. The tracheal system is complete, with spiracles on the anterior margin of the 1st thoracic segment and on the dorsum of the 7th abdominal segment. The anterior spiracles are pointed, dart-like structures, heavily sclerotized and set in pits, while the posterior pair are distinctly dorsal and are in the form of very heavy, dark hooks directed toward the head. The latter are apparently completely closed, and the anterior pair is not open until late in the stage (Clausen 1940/62).
At first the puparium is a pale yellow, but finally becomes black. There are 10 distinct segments, and the dorsally located operculum extends to the posterior margin of the 3rd segment. The anterior spiracles are terminal in position when fully extended. The tails remain attached to the puparium, but they are shrunken and brittle. The prothoracic spiracles of the pupa are internal and do not protrude through the integument.
First instar larvae derive their food directly from the host's body fluids, which are absorbed through a delicate integument. The third stage feeds on the fat body, and gross feeding occurs in the final stage. The larva is virtually incapable of movement prior to the 3rd molt. Before pupation, when the host's body contents have been largely consumed, the anterior spiracles are extruded to their full extent and forced through the host integument, usually at the lateral margin. The host integument then dries and closely envelops the puparium. Because of this, the skin is broken at the time the puparial operculum is raised, allowing the adult fly to escape.
There are 1-6 individual flies able to attain maturity in each host. The life cycle takes ca. one month, and 5-6 generations occur per year in California.
Vayssiere (1926) and Thorpe (1934) studied another species, Cryptochaetum grandicorne Rond. This is a solitary internal parasitoid of Guerinia serratulae F. in Europe. There are several differences in the morphology of the immature stages and in the manner of development when compared to C. iceryae. There are only 3 larval instars rather than 4. The life history is well adapted to the cycle of the host, and the pupal period of 6 months or more covers the time in which the host is in the dormant phase. Oviposition occurs only in 1st instar hosts after they have become fixed on the food plant (Vayssiere 1926, Thorpe 1934).
The egg is longer and more curved and the anterior end is relatively wider than in C. iceryae. The increase in size during incubation is much less, and the 1st instar "embryo" larva is more elongate and may be distinguished by a pair of unpigmented mandibles projecting from the open mouth. The 2nd instar larva has 11 body segments and bears a transverse row of digitate spines on the dorsum and sides of the 3rd thoracic segment and 4 rows, completely encircling the body, on each of the following 8 segments. The tubular tails increase much in length during the 2nd stage. The tracheal system is similar to that for 3rd instar C. iceryae. This instar may persist for 3-4 months. The 3rd instar larva has the tail filaments shrunken and often broken off, so that in the latter part of the period they may be shorter than the body. The anterior spiracles are palmate and lightly pigmented, while the posterior pair show an opening near the base of the spine. Just before pupation, both pairs of spiracles are thrust through the host integument. Vayssiere (1926) found that the hook-like posterior spiracles of the 3rd instar larva are fixed in one of the large tracheae of the host, but the occurrence of this habit was not found by Thorpe (1934).
The biology and behavior of Cryptochaetum sp. parasitic in Drosicha corpulenta and others of that genus in Japan differ in some respects from the two species just discussed (Clausen 1940/62). Adult flies feed mainly on honeydew secreted by Kermes miyasakii Kuw., which occurs on the same trees as the host insect. There are two generation per year, and the summer brood of females oviposit in the young scales, passing the winter as young larvae within the living hosts. In this generation both male and female scales are parasitized, with development being completed in springtime. These parasitized scales usually do not leave their hibernating sites in crevices in the trunk of trees. Male hosts are in the prepupal stage at the time of death, and emergence of the spring brood of Cryptochaetum coincides with that of male hosts, which is usually 10-25 May in central Japan (Clausen 1940/62).
Cryptochaetum sp. eggs in Japan measure 0.35 X 0.08 mm., and are thus much larger and more elongate than the eggs of either C. iceryae or C. grandicorne. The mature larva is 3.2-4.0 mm. long, with the caudal filaments measuring 4.0-5.5 mm. The latter are uniformly thick for their entire length, except of the basal portions, which are bulbous. The puparium is a deep red, which shows through the host's integument. The tails remain unbroken nd turgid even after emergence of the adult fly from the puparium.
Clausen (1940) noted that the life cycle given above, in which the parasitoid passes through two generations annually on the same host generation, is much different from that given by Vayssiere and Thorpe for C. grandicorne, which has only a single generation annually and which provides for the long summer period when hosts are not available, by undergoing a prolonged pupal diapause.