Description & Statistics
The Cyrtidae is a small family which is rarely encountered in the field. Adult flies are distinguished by the large humpbacked thorax, the rather small head, situated markedly ventrally with respect to the thorax, and frequently by a pronounced swollen appearance of the abdomen. The proboscis is long in several genera, where it is used for sucking the nectar of flowers. In most forms the proboscis is reduced, and feeding is thought to be rare. In some species the flies are exceedingly rapid in flight, although in most they are sluggish and may be recognized by an uncertain and undulating movement. They are clumsy in walking, as if their slender legs were inadequate to support their bodies. Wings are long, and their tips usually drag on the surface while walking. Adults of many species can be easily captured while resting or ovipositing. All species are believed to be internal parasitoids of spiders, mainly of the Clubionidae, Lycosidae and Salticidae (early records of species occurring as predators in egg sacs are thought to be in error (Clausen 1940/62)). Host preferences of Cyrtidae were noted by Millot (1938), these representing 7 families, the majority of species being arboreal. The different cyrtid species seem to have a wide host range, each attacking representatives of several families. Extended studies on behavior are those of King (1916) on Pterodontia flavipes Gray, a solitary internal parasitoid of lycosid and epeirid spiders, and by Millot (1938) on Ogcodes pallipes Latr.
Biology & Behavior
Oviposition.-- Oviposition has been observed in Ogcodes brunneus Hut (Maskell 1888), O. costatus Loew (Gillett 1924), O. pallipes Latr., O. varius Latr. (= fuliginosa Er.), Opsebius diligens O.S. (Cole, 1919), and P. flavipes. F. Stein in 1849 noted that eggs of Ogcodes varius Latr. were laid in rows on Equisetum. Females of O. brunneus were found crawling over apple twigs and depositing eggs in such numbers on the bark to cause the latter to be black as found with sooty molds (Clausen 1940/62). The eggs of an Ogcodes sp. were deposited largely on dead twigs (Konig 1894). O. pallipes of Europe lays its eggs in irregular rows on foliage and bark of various trees and also on Equisetum A female of O. costatus was observed to fly into a house where she laid her eggs in irregular rows on a white cloth (Gillett 1924). King (1916) observed P. flavipes to oviposit while in flight, which seems to be normal for that species. The female hovers on the leeward side of a tree trunk, fence post, etc., only a few inches from it and projects her eggs against the surface with great rapidity. Being partly covered with mucilage, they adhere to any surface that they strike. Opsebius diligens has been observed to oviposit while in flight in a caged situation.
Clausen (1940) concluded that the majority of species lay their eggs on plant surfaces, principally the twigs and branches, while the females are resting or crawling about on the surface. In several species there seems to be a tendency for females to congregate during oviposition, and thus the plants or branches of trees used for this purpose become encrusted with large numbers of eggs, making them conspicuous because of the black color. Oviposition is apparently entirely independent of the presence of spider hosts, although naturally the flies occur in favored spider habitats. The rate of oviposition and total number of eggs laid by Cyrtidae are high. All species produce several thousand eggs, laying them in short order. A female of P. flavipes laid 2,300 within 45 min., and a total of 3,977 was obtained from another individuals. About 3,000 were laid by a single female of Ogacodes costatus during 4 hrs. O. pallipes lays a total of 2,000 eggs at the rate of ca. 1 per min., during her periods of activity. It is probable that the egg capacity of some species reaches 10,000 (Clausen 1940/62).
In Cyrtidae, as is true of Trigonalidae in the Hymenoptera, eggs are not incubated at the time of laying, which is in contrast to the fully developed condition of the embryos in microtype eggs of some Tachinidae. The incubation period is relatively long, being 32-33 days in Pterodontia, 49 days in Opsebius diligens, and 30-45 days in Ogcodes pallipes. The range in the last species is due to different temperature conditions. These eggs hatching after an incubation period of definite duration also distinguishes them from the microtype eggs of Trigonalidae and Tachinidae, which must be eaten by the primary or secondary host before hatching can occur. Clausen (1940) commented that thus the necessity for the egg to be eaten by the host was replaced by another, involving search for the host by the 1st instar larva. The reproductive potential of the three families mentioned are about equal, which if a reflection of survival hazards, indicates that the losses under both sets of conditions are about equal.
The young larvae or planidia cut away or force off a well defined circular cap at the smaller, anterior end of the egg during hatching. This kind of hatching is comparable to that of the dehiscent macrotype eggs of certain Tachinidae.
Larval Behavior.-- First instar larvae are of the planidium type, and are able to undergo a free living period of considerable duration without food. Locomotion is either by a looping motion or by jumping. In the latter case, the larva of Pterodontia stands erect on the caudal disk, which is specialized for the obvious purpose of giving a firm hold on the substrate, and projects itself into space by the sudden downward thrust of the long caudal cerci, bringing them into line with the longitudinal axis of the body from a position at right angles with it (Clausen 1940/62). However, Bovey (1936) found this to be accomplished in O. pallipes by the lowering of the head to the substrate near the point of attachment of the caudal disk, thus forming almost a circle, following which the body was abruptly straightened. This action simulates that of a released spring. The planidia of Opsebius are able to crawl in a looping manner along a single strand of a spider web (Cole 1919). Jumping does not seem induced by the host presence, but it serves to accomplish dispersion. Actual host contact is mostly by chance, and the planidia, when alert and awaiting a host or stimulated by its presence in the immediate vicinity, assume an erect position (Clausen 1940/62).
It is not know exactly how long the planidia are able to life if contact with a host is not made. One weeks seems to be the maximum, which is short in comparison with that for planidium larvae of other groups of insects. The planidia enter the bodies of their hosts probably through the thin membranes of the articulations of the legs. While doing so they cause appreciable irritation to the host spiders, and Theridium adults bearing planidia of Opsebius scratched themselves excitedly, seemingly being able to kill some of the parasitoids with their mandibles (Cole 1919).
King (1916) recorded finding the planidia of Pterodontia in all parts of the host body, even in the legs and palpi, although the majority were in the thorax. They persist in this stage, without much growth or direct relation to any host organ, until springtime. Millot (1938) found Ogcodes pallipes young larvae in the abdomen during winter, but they migrate to the respiratory organs in the early spring and the first molt follows. Part of the second stage is also passed in the abdomen, and the second molt occurs there. The larva of Astomella lindenii Er., while in the host abdomen, derives air from one of the lung chambers of the host by placing the caudal spiracles against the lung wall and seemingly making a minute perforation (Brauer 1869a). This also occurs in O. pallipes, although Millot (1938) found some individuals to perforate the abdominal wall instead and thereby make direct connection with the outside air. In the latter, there is evidence of the formation of a respiratory funnel which is comparable to many Tachinidae.
Third instar larvae complete their feeding quickly and consume the entire contents of the host abdomen. Emergence is through a hole cut in the ventral abdominal wall. The host spider usually shows no evidence of parasitism, either by modification in body form or a change in activities, until just a few hours before it dies. However, the spider does spin a mat of silk just before death, and Locket (1930) noted that this was done even by male spiders, which was abnormal. He attributed this action in both sexes to increased pressure within the abdomen from the large parasitoid body; a measure of relief is probably obtained by expelling the silk.
Pupation.-- Pupation is outside the host body, in either the web or the burrow or in some sheltered place near the place of host demise. Emerged larvae of Acrocera fasciata Wied. suspended themselves in the host web by clinging to a strand with their mandibles; they were supported also by strands beneath the body (Emerton 1890). A sticky body surface aids the mature larvae to retain their positions in the web or on a trunk. Actual pupation usually occurs the day following emergence from the host, and the meconium is cast by the prepupa. The pupal stage is short, being 7 days in Pterodontia, 6-11 days in O. pallipes, and about the same in several other species in summer. Adult emergence occurs through a median longitudinal break over the prothorax of the pupal shell. There seems to be only a single generation annually, 9 or more months being passed in the first larval stage within the active or hibernating spider.
For a description of immature stages of Cyrtidae please refer to Clausen (1940).
Clausen, C. P. 1940. Entomophagous Insects. McGraw-Hill Book Co., NY. & London. 688 p.