Colobaea americana Steyskal, 1954

Bratt, Albertus D., Knutson, Lloyd V., Murphy, William L. & Daniels, Anthony A., 2020, Biology, immature stages, and systematics of snail-killing flies of the genus Colobaea (Diptera: Sciomyzidae), with overviews of aspects of the tribe Sciomyzini, Zootaxa 4840 (1), pp. 1-64 : 18-20

Colobaea americana Steyskal, 1954

Map 2

(BNN 6512, 6551, 6601, 6608, 6611, 6627)

Colobaea americana is the smallest known species of Sciomyzidae , with field-collected adult flies ranging from only 1.8 to 2.0 mm in length and with some laboratory-reared adult flies measuring only 1.2 mm in length. The species is similar to C. distincta , C. pectoralis , and C. punctata in color pattern and in development of the dorsobasal aristal setulae. Steyskal (1954) figured the antenna and wing but did not describe or figure the male postabdomen, which has not yet been figured, although both the male and female abdomens were described in detail by Murphy et al. (2018).

In C. americana , the only species of Colobaea for which chromosomes have been numbered, the diploid number of chromosomes is 12 ( Boyes et al. 1969), with five metacentric pairs of autosomes. The X allosome is rather small. This configuration is similar to that of some species of Parectinocera Becker, 1919 and Pherbellia but unlike that of any examined species of Pteromicra Lioy, 1864 , in which the diploid number is 10.

Colobaea americana was described from four specimens from Aylmer , Quebec, and Aweme , Manitoba, both in Canada. In New York, USA (at Cobleskill , Geneva , Ithaca , and Spencer ), ADB and LVK collected puparia that subsequently produced adult flies. C.O. Berg (in litt.) collected others at Lacombe, Alberta, Canada. The only records of field-collected adult flies are 31 July (Aylmer, Quebec) and 11 August (Aweme, Manitoba) ( Steyskal 1954), but Tom Murray (Groton, Massachusetts, USA) posted two photographs that he took in the field on 16 May 2005 in Harvard, Worcester County, Massachusetts, of a live C. americana resting on a leaf .

The habitat distribution of C. americana is rather broad, as is that of its most frequent snail prey, Gyraulus parvus (Say) . The collecting sites at Cobleskill, Ithaca, and Spencer, New York, and at Lacombe, Alberta, included small, permanent, exposed ponds. A vernal swamp at Geneva, New York, was extremely productive for collecting puparia in floating snail shells. The Geneva locality, Savage Road Swamp, is described in detail in Bratt et al. (1969); all laboratory rearings were initiated with adult flies that emerged from puparia collected there during 1962, 1965, and 1966 (FT 6206, 6501–02, 6512, 6551, 6601–02, 6606–07, 6611). Adult flies usually mated on the day of emergence and continued to mate for as long as 10 days. While mating, the male positioned his foretarsi on the parafrontal areas of the female’s head or on the anterolateral surfaces of her thorax, held his midlegs above her wings, and laterally grasped her postabdomen with his hind tarsi. Adult flies occasionally consumed the mucus of living snails in addition to the mixture of honey, yeast, and powdered milk with which they were provided. The preoviposition period was 4–7 days. In the laboratory, adult females oviposited throughout their lives, laying eggs in no apparent pattern on damp moss or cotton, never on living or dead snails. The incubation period at room temperature was about 24 hours.

Larvae appeared to prefer the snail G. parvus as prey. Many puparia were found in shells of G. parvus at Geneva, New York, and a few puparia were collected in G. parvus shells at Cobleskill and Spencer, New York, and at Lacombe, Alberta. One puparium was found in the shell of a Physa Dreparnaud sp. (J.L. Bath, collector). During laboratory rearings, larvae killed and consumed G. parvus but did not attack Helisoma trivolvis (Say) or Physa sp.

Unlike larvae of many other species of Sciomyzini , newly hatched larvae did not crawl about on the shell or exposed soft parts of the snail and feed on mucus. Instead they immediately inserted themselves between the mantle and shell of the snail, which invariably reacted by retracting into its shell until its soft parts were about one-third of a whorl from the aperture. Each first-instar larva subsequently remained between the mantle and shell, with its posterior spiracles barely exposed. Usually only one first-instar larva, but occasionally as many as three, attacked an individual snail. Each snail infested with one larva lived for about two days after invasion. After the snail had died, the larva continued to feed, consuming almost all of the decomposing snail tissues. A single G. parvus of about 3 mm in diameter appeared to provide enough nourishment for the development of a larva from hatching to pupariation. If a snail penetrated by a newly hatched larva was considerably smaller than 3 mm, the larva subsequently killed and consumed a second snail. During laboratory trials, larvae were unable to subdue snails 5.5 mm or greater in diameter. The total duration of the larval stage was 5–6 days.

All puparia were formed in snail shells. Before pupariating, larvae pushed any remaining decayed tissue to the aperture of the shell and formed a thin, plate-like encrustation of a small amount of calcareous material at the aperture. The origin and formation of this structure are similar to that involved in the production of “septa” by pupariating larvae of certain species of Pherbellia (Knutson et al. 1967, Bratt et al. 1969). As in larvae of some species of Pherbellia , the Malpighian tubules of mature larvae (where the calcareous material apparently is produced/stored) of C. americana appeared very large and white, but, unlike the situation with Pherbellia larvae, those of C. americana formed the septum while still in a feeding position, i.e. with the anterior end deep in the shell and the posterior end near the aperture. The difference in positions of the larvae at the time of excretion of the septum material probably is related to the fact that whereas larvae of Pherbellia produce a relatively large amount of septum material, larvae of C. americana produce only a small amount, retaining most of it. Larvae of Pherbellia can “afford” to expend some of the material in displacing it from the inner whorls of the shell to the aperture by peristaltic body movements, but larvae of C. americana apparently cannot. Septa of C. americana were formed at the aperture of the shell, not several millimeters inside the aperture as with species of Pherbellia . Some of the remaining snail tissue that the pupariating C. americana larvae pushed to the aperture usually became incorporated into the septum. The function of the septum is unknown; it might ensure successful overwintering by aiding in flotation of shells containing puparia or it might serve as a physical barrier against potential predators.

In the laboratory, many puparia were formed between 20 January and 6 June. The puparial period lasted 9–11 days. Adult flies emerged between 29 January and 17 July. Laboratory-reared males lived 11–47 days. Females lived 16–60 days.

Colobaea americana apparently overwinters in the puparium. Puparia were found in the field at Geneva, New York, on 11, 14, 27, and 28 April and on 6 November. The puparia collected during April and held at room temperature produced adult flies 10–23 days later. Astonishingly, from puparia collected 6 November 1963 and held at 5°C until 20 December 1965, adult flies emerged 9–19 days after being returned to room temperature, having spent 775 days in diapause. Seasonal development of the species apparently is limited only by low temperatures. This conclusion also is supported by the fact that five consecutive generations were produced in the laboratory at relatively stable ambient indoor temperatures.

Hymenopterous parasitoids of Ichneumonidae and Pteromalidae were reared from puparia collected at Geneva, New York.

Seven Orthizema n. sp. ( Ichneumonidae ) (determined by R.W. Carlson, Systematic Entomology Laboratory, Agricultural Research Service, U.S. Department of Agriculture) emerged between 18 and 21 May, one each from seven puparia collected 27 April. Two Eupteromalus sp. ( Pteromalidae ) (determined by B.D. Burks, same affiliation as R.W. Carlson) emerged 27 April and 14 May, one each from two puparia collected 14 April, and another emerged 11 May from a puparium collected 28 April.

Figures

MAP 2. Geographic range of Colobaea americana Steyskal.