Worcestershire Record No. 4 May 1998 p.4
By David J L Harding
The genus Enoicyla is unique among European Trichoptera in having flightless females and terrestrial immature stages. Three species are known in continental Europe (Fischer, 1973) of which only one, E. pusilla Burmeister, occurs in Britain, where it is commonly referred to as the terrestrial or land caddis.
After the first British records of larvae of E.pusilla, found by Mr. Fletcher of Worcester in moss near the roots of willow trees, observers were asked to keep a look-out for this peculiar insect (McLachlan, 1868). Despite the diligence with which Victorian entomologists doubtless scoured their local woodlands, the only reports during the next decade seem to have been by McLachlan and Fletcher, still in Worcestershire. However, these were from woodland, rather than from the margins of rivulets, and therefore more comparable to continental records. According to Hickin (1965, 1967) it was not discovered again until April 1957, when he was camping in a meadow on the Worcestershire edge of Wyre Forest, near Bewdley. Ironically, after many years of fruitless searching in Wyre, Norman Hickin was pipped at the post by his daughter Sari, who first spotted the larvae on her tent. Thereafter, the species was claimed to be restricted to a small area around Wyre Forest railway station (Hickin, 1971).
During the 1970s I learned of other sites, including Chaddesley Woods NNR, (between Kidderminster and Bromsgrove), Shrawley Woods, (dominated by small-leaved lime, south of Stourport), West Malvern and various sites in Herefordshire, including Bromyard Downs. My most north-westerly find so far has been from just to the east of the Clee Hills, and in 1991 I found my first Staffordshire colony, albeit a few hundred yards from the Shropshire boundary, near Enville. On the basis of these records I was quoted by Wallace (1991) as stating that E. pusilla is confined in Britain to an area which I refer to as 'Greater Wyre Forest' (i.e. within a 10-mile radius of Bewdley). Consequently this species is categorised as rare (Red Data Book Category 3) by Wallace (1991), being 'common within its restricted range'.
I am now appealing to readers (and their families) to help fill in gaps within this area of the West Midlands and, optimistically, to extend its recorded distribution to other parts of Britain. To assist this quest, I will summarize the life cycle and relevant aspects of behaviour, before suggesting were and when larvae are most likely to be found.
Phenology of E. pusillaAdults only live for a few days. Winged males are occasionally seen at the end of September or the beginning of October. Each female lays about 50 eggs, typically amongst moss at the base of a tree. These hatch after about a month, so that at Shrawley first instar larvae are first found at the beginning of November. Morphologically, the larva is a typical limnephilid, with the exception of gills, which are absent, and of the lateral protuberances on the first abdominal segment, which are less well developed than in aquatic species. After hatching, the young larvae construct conical cases, mainly from organic matter, and begin to feed on mosses, algae and tree-leaf litter. By the New Year, most have undergone their first moult, having incorporated sand grains into the anterior rim of the slightly curved case (see illustration). Larvae can be separated into five instars, based on head widths, although there is some overlap between instars IV and V. Population densities can exceed 15OO per square metre in December, but may be less than 10 per square metre by July. Peaks for successive instars occur at roughly monthly intervals between December and April. Average case lengths range from 1.8mm in November to 8.2mm in June, with a maximum value of 10mm
As the season progresses, some of the larvae disperse away from the bases of trees. Under moist conditions larvae of any instar can be seen moving around, like Allbran, feeding among the surface layers of the forest floor or on mosses and algae on tree trunks at heights of up to about a metre. Sometimes they hang from twigs and other vegetation, apparently drying out after being soaked by rain or dew. During January 1993 there was a mass migration of larvae onto bramble bushes, and the invasion of the Hickins' tent may have a similar explanation.
Kelner-Pillault (1960) observed comparable behaviour after heavy rain; as the relative humidity fell below 70%, so larvae descended into the litter again. We have shown that larvae can survive for several days in the laboratory at humidities down to 60%, and that they appear to be much happier in cultures of dry litter than e.g. oribatid mites. In the field, if the weather is particularly cold or dry, larvae retreat deeper into the forest floor.
As the numbers of living larvae decline during the summer, so the abundance of empty cases increases, perhaps due to predation. In June, larvae retire into the forest floor and seal off the posterior opening of the case with a silken operculum. They can still move around, but they do not feed. In August they seek a more permanent shelter in soil or in humus-filled tree holes. Here they pupate, having first sealed the anterior opening.
Feeding habits and habitatsE. pusilla is a typical detritivore, feeding predominantly on fungus-conditioned tree leaves (Packham et al., 1992). In culture, larvae will accept broadleaves, conifer needles, bracken and bramble leaves, as well as mosses and algae, but not grasses. Although larvae may be found in grass tussocks, they must have access to tree leaves, so that this species is restricted to woodland. They have been found in coniferous sites, including larch and Scots pine, but I associate them typically with oak or lime woodlands on well drained, slightly acid soils. Case composition varies with habitat in terms of the relative proportions of tree litter and sand grains.
Advice on samplingOnce you have got your eye in (and your knees in damp litter) you can spot even the smallest larvae (ca. 2mm) at the bases of trees. Sieving of litter into a white tray may help at this stage. Later, larger instars are easier to see, but are less abundant. The ideal conditions are sunshine after overnight rain any time between February and April, when larvae should be active on the litter surface and even on tree trunks. During the spring and summer, family outings could utilize the keen eyesight and treasure-hunt instincts of children. Don't forget that old cases can be found at any time.
AcknowledgementsAmong numerous helpers in the tracking down of E. pusilla, I would particularly like to thank Noel King, Malcolm Clark, Mike Shotton, Basil Miles, Ian Wallace, the late Norman Hickin, Pam Copson and Cezary Tomaszewski, as well as various project students.
|Fischer, FC. (1973). Trichopterum Catalogus, 15: 206-219. Nederlandse entomologische. Vereniging, Amsterdam.
||Hickin, NE (1965). Forest Refreshed. Hutchinson.
||Hickin, NE (1967). Caddis Larvae. Hutchinson.
||Hickin, NE (1971). The Natural History of an English Forest: the Wild Life of Wyre. Hutchinson.
||Kelner-Pillault, S (1960). Biologie, écologie d'Enoicyla pusilla Burm. (Trichoptères Limnophilides). L'année biologique 36: 51-99.
||McLachlan, R (1868). Occurrence in England of the larva of a terrestrial Trichopterous insect; probably Enoicyla pusilla Burmeister. Ent. Mon. Mag. 5:43-44.
||Packham, JR, Harding, DJL, Hilton, GM & Stuttard, RA (1992). Functional Ecology of Woodland and Forests. Chapman and Hall.
||Wallace, ID (1991). A review of the Trichoptera of Great Britain. Nature Conservancy Council.
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