Immature Stages of Meloidae
Clausen (1940) remarked that he eggs of the different species of the Meloidae are quite consistent in form, being somewhat cylindrical and three or four times longer than wide, and with both ends smoothly rounded. The chorion is delicate and unsculptured, and the color ranges from white to yellow and orange. The size of the egg is governed not only by the size of the beetle itself but by the number produced by the particular species. Thus, in a species that produces 3,000 to 5,000 eggs, they are markedly smaller than in one that deposits only a few hundred.
The larvae of the Meloidae pass through a rather striking metamorphosis in which six instars are usually recognizable. Riley (1883) proposed that they be designated as follows:
Caraboid (Carabidoid)‑‑second‑instar larva
Scarabaeoid‑‑third‑ and fourth‑instar larvae
These distinctive forms are readily recognizable in all species which develop in locust‑egg masses, but the later instars are not so well‑differentiated in those which attack bees. Among the latter, the coarctate larva has been variously termed the pseudolarva, pseudonymph, pseudopupa, and pseudochrysalis.
The first‑instar larva has attracted the interest of entomologists for a long period of time, and for many years its identity was not established. It was at first considered to be an adult insect, externally parasitic upon bees; it was named Pediculus apis by Linnaeus, and later the genus Triungulinus was erected for it by Dufour. The term "triungulin," later modified to "triungulinid," has consequently been used to designate the larvae of the family; the reference is to the three claws borne at the end of the tibia in the forms described. This character is not constant throughout the family as pointed out by Cros (1917) and Böving (1924), there being a wide variation among the different species, and the term is not applicable to a considerable portion of the family. Several later authors have used the term "primary larva" instead, but this is likely to lead to confusion, for it generally indicates a host‑parasite relationship. In all essential respects, this larva is of the true planidium type, having the distinctive features and habits of larvae of the Perilampidae, Eucharidae, Cyrtidae, Ripiphoridae, and Strepsiptera.
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The body of the first‑instar larva (Figs. 239A, 240A) is fusiform, is broadest in the thoracic region, and has the nine abdominal segments successively narrower. It is white, yellow, or orange at the time of hatching; but the integument quickly darkens, often becoming almost black. The head and thorax combined are approximately equal to the abdomen in length. In outline, the head varies from triangular to quadrangular. The black ocelli, either simple or double, are situated nenr the lateral margins. The mandibles range from the simple falcate form to those having up to 12 teeth, the more strongly dentate forms being of species that attack locust egg masses. The antennae are large, cylindricnl, and three‑jointed; the second joint is longest and bears a sensory organ, and the third terminates in a spine that may vary in length from half to six times that of the entire antenna. The thoracic segments arc long and heavily sclerotized, and the legs comprise four principal joints, with the tibia terminating in a simple claw‑like tarsus, with a pair of setae of varying size at the base, or in a "trident of Neptune" structure, to which the term "triungulin" refers. The latral processes, however, have no muscle attachments.
The abdominal segments in many species have the tergal and pleural plates hcavily sclerotized and indurate, whereas the skin of the intersegmental areas is delicate and white in color. The body consequently has a distinctly banded appearance after becoming distended with food. In many of the species that attack locust‑egg masses, this coloration of the segments is not nearly so distinct. One pair of caudnl cerci is found usually, though not always, on the ninth abdominal segment, these being very short in some species whereas in others their length may exceed half that of the abdomen. In a few species, a supplementary pair is found on the eighth segment. The various body segments may bear a number of short setae, usually arranged in transverse rows. The abdomen terminates in a somewhat bulbous adhesive disk that is utilized in clinging to the substratum. By means of the erectile caudal cerci, aided by the adhesive disk, the triungulinid is able to assume an upright position when stimulated by the approach of a carrier, a habit that is general nmong larvae of this type in several orders.
The data available indicate that the number of pairs of spiracles may be seven or eight, with the first pair occurring on the mesothorax and the remainder on the successive abdominal segments. In a number of species those of the mesothorax and the first abdominal segment are very large, and the remainder minute. Several species are known to have the spiracles of the eighth abdominal segment borne at the tips of a pair of falciform processes, which are considered to provide against the entry of fluids into the tracheal system.
The second‑instar (caraboid) larva (Fig. 239B) differs from the preceding instar principally in its more robust body and reduced head and legs. A rather delicate, white skin replaces the heavily sclerotized and indurated integument. In Tricrania (Fig. 24OB) and others that attack bees, the body assume a distinctive boat‑like form in which the dorsum is nearly flat and the venter markedly convex, enabling it to float readily upon the surface of the food material in the cell. This modification in form is accompanied by a shift in the position of the spiracles, which become markedly dorsal. The antennae are now somewhat conical in form and lack the long terminal setae. The integumentary setae may persist, particularly in the species attacking locust egg-masses, but they are reduced in size.
The third‑ and fourth‑instar (scarabaeoid) larvae (Fig. 239C) of the species attacking locust eggs are quitc similar in all respects and have a striking resemblance to the grubs of Scarabaeidae. The abdomen is much increased in size, and the legs are further reduced. In Tricrania (Fig. 240C), Apalus, and others of the Nemognathinae, the third instar is quite similar to the second, retaining its boat like form, but in the fourth instar the abdomen is greatly distended and markedly convex dorsally as well as ventrally.
The fifth‑instar (coarctate) larvae of Epicauta (Fig. 239D) and others of similar host preferences are markedly different from the preceding instars in having the legs rudimentary, represented by conical protuberances only, the mouth parts likewise rudimentary and not serving for feeding purposes, and the body segmentation often indistinct. A pronounced longitudinal ridge extends along the lateral margins of thc first seven abdominal segments. The integument is much heavier than in the preceding instars and bears transverse corrugations. Among the species of Nemognathinae and other groups attacking bees, this instar (Fig. 240D) is not nearly so distinctly differentiated, though the mouth parts and legs are greatly reduced and functionless. The term "coarctate" is not properly applicable to all larvae of the family of this instar, inasmuch as many are not enveloped by the exuviae.
The sixth‑instar (scolytoid) larva is so named because of its resemblance to the larvae of the Scolytidae, and it thus represents a reversion to a form somewhat similar to the scarabaeoid. It is of reduced size as compared with the two preceding instars, owing to the last two transformations having been accomplished without feeding. The legs, though still largely functionless, are relatively larger than before.