A Contribution to the Life Histories of Two Fungus Gnats, Scythropochroa nitida Edw., and Sciara annulata Mg., (Diptera, Mycetophilidae, Sciarinae)

By Baughan Wisely
Zoology Department, Canterbury University College, Christchurch

ovoid, and measured 419 × 283μ (average of 10; range 423–387 × 297–270μ). The chorion was raised into slight ridges, often forming pentagonal cells (Fig. 2) dotted with minute pits. Hatching began after 28 days at approximately 17° C., the larvae on emergence averaging about 1.0 mm in length.

The egg masses of S. annulata were found in similar conditions but were smaller (about 1.0 mm in diameter). Two newly-emerged females contained 41 and 42 eggs respectively, and the number of eggs in two masses was 35 and 46. The eggs (Fig. 3) were smaller than those of S. nitida, measuring 261 × 144μ (average of 10; range 279–254 × 180–153μ). Hatching was not observed.

Larvae

The larvae (as in the case of Arachnocampa luminosa and several other local fungus gnat larvae) were found in a water saturated habitat. They were only found in those portions of the log that were waterlogged to such an extent that slight pressure caused exudation of drops. Larvae of S. nitida (Fig. 4) measuring 4.0–5.0 mm in length were found in burrows in the log and identified by comparison with larvae bred from identified parents. In the following description Keilin's (1919) terminology is used where applicable.

Head capsule (Figs. 5, 6, 7) swollen at sides and emarginated postero-dorsally with two lateral plates, (lp), a median plate (mp) and a transparent labrum (lab). Edges of lateral plates adjacent to median plate bearing four pairs of circular pits (cps) of which the third pair from the posterior end is the largest. Labrum (Fig. 8) with curved chitinous rod (cr) bearing an anterior projection (ap) and a branch on each side (crb) terminating in 5–6 teeth. Antero-lateral corners of labrum each bearing a chitinous plate (alp) with 4–5 teeth. In some cases small pits or cones were seen in the positions indicated. Mandible (Fig. 9) with four (1–4) prominent apical teeth and a clump of setae (cs) on the inner proximal corner. Maxilla (Fig. 10) with basal “cardo” (c) maxillary palp (mpx) with single basal seta (bs) and 1–2 distal columnar setae surrounded by 5–6 pits. Outer part of maxilla transparent, inner part bearing five (1–5) prominent and several lesser teeth. Antenna (Fig. 14) consisting of a small cone with basal pits situated in the posterior edge of a prominent chitinised ring in the head. (The labium, a V-shaped chitinous structure at the bases of the maxillae, was not observed satisfactorily.) The main differences between S. nitida and S. annulata larvae are as follows:—

In S. nitida larvae the alimentary canal consists of a buccal cavity and pharynx situated anteriorly in the head and connected by a narrow oesophagus to the proventriculus in the metathoracic region. The salivary gland ducts fuse and enter the buccal cavity by the base of the mandibles, the opening being strengthened by a chitinous ring. Immediately posterior to the proventriculus two anterior caeca leave the midgut and accompany it for the greater part of their length. Numerous gregarine parasites were noticed in and alongside this region in large larvae. More posteriorly four malphighian tubules leave the gut at the mid and hind gut junction.

The hindgut then loops through 180° before dilating into the rectum just prior to the anus. The respiratory system is peripneustic with 8 pairs of spiracles. As in other Mycetophilidae larvae, the body consists of 12 segments, excluding the head.

According to Imms (1951: 647) Sciarinae larvae feed on a variety of food. Some have been found in decaying apples, pears, turnips and potatoes, and others attack seedlings and mushrooms. Both of the species described here fed and grew on rotting willow wood.

Pupae

Numerous pupal exuviae were found projecting well out from the surface of the upper, drier parts of the log, and many of these (as evidenced by the antero-dorsal splits) had liberated imagines. Similar exuviae were not observed on the surface of the water saturated portion of the log, but they were found in larval burrows several centimetres within the log. Two S. nitida larvae, quiescent and beginning to pupate, measured 10.0 mm each in length and shortened to 5.0 mm during pupation. Six S. annulata larvae averaged 4.0 mm before, and 2.0 mm after pupation; this process lasting five days at approximately 18° C. In both species two curved bristles appeared on the vertex and minute spimes appeared posteriorly, particularly on the dorsal aspect. A S. nitida pupa is shown in Fig. 17.

Imagines

Tonnoir and Edwards (1927) have already enumerated the features that distinguish the Mycetophilidae from other families of Diptera and have also given the characteristics of the subfamily Sciarinae and the genera Scythropochroa and Sciara. The wings of the Purau Valley imagines were in the unexpanded condition shown in Fig. 23; they were soft, contained fluid, and were relatively shorter in length than those of museum specimens Nevertheless they possessed the same essential venation as given by Tonnoir and Edwards (Fig. 24). Further confirmatory points in the case of S. nitida included the presence of the three gray dorsal stripes on the thorax and the uniformly hairy scutellum; and in S. annulata the large apical spine of the male clasper. Edwards' original description (1927: 799) of S. nitida (? Mycetophilidae anarctica Hudson) was based on a female. The male genitalia (Figs. 20–22) have not been described hitherto and differ from that of other recorded New Zealand Mycetophilidae in that the terminal joint of the clasper carries 7–8 stout spines forming a rake-like structure (Fig. 22 r).

Such imagines can reproduce successfully without leaving their larval habitat. Imagines were found moving around freely in freshly opened larval burrows; their short flexible wings did not prevent them turning when they reached a blind end. Copulations were frequent and numerous females were discovered depositing egg masses which were found by rearing observations to be fertile.

Since the high humidity present in the burrows and rearing jars did not facilitate the diving of wings, freshly emerged imagines of both species were placed in dry containers to see if expanded wings would result. All such attempts were unsuccessful; the wings dried in the unexpanded condition. However, when the lid was accidentally left off a jar of wet wood containing only mature S. nitida larvae, imagines of both sexes began flying out several days later. No significant difference could be found between these and Tonnoir's specimens. This was repeated with more larvae in uncorked 3in × 1in tubes with the same result, but could not, through a shortage of suitable larvae, be tested for S. annulata.

This laboratory evidence suggests that a gradual drying out of the habitat (which would ultimately make it unsuitable for larvae) tends to favour the production of flying imagines of S. nitida, capable of colonising new habitats. One of the factors connected with this may be the rate of water loss during pupation. Whatever the mechanism it is clear that in both species there are, in addition to the flying forms.