Functional feeding ecology in Central European species of subfamily Drusinae (Insecta: Trichoptera)

Abstract

The functional feeding ecology of Drusus muelleri, D. nigrescens, D. melanchaetes D. franzi and D. alpinus is discussed and compared with feeding modi of other Central European Drusinae.

1 lntroduction

Based on morphological characters, molecular genetics (mitochondrial and nuclear sequence data) and feeding ecology (functional mouthpart anatomy) Central European Drusinae larvae are investigated in order to construct a phylogenetic tree, to test contemporary species grouping concepts and to describe the hitherto unknown larvae of the 24 Central European species. In the following, we deal with functional feeding ecology of these species.

2 Results and discussion

So far, 21 species of the 24 Drusinae species in Austria, Germany and Switzerland are known in their larval Stage (Tab. 1). Drusus alpinus, D. franzi, D. melanchaetes, D. muelleri and D. nigrescens have been newly described within the project to date (Graf et al. 2005, Waringer et al. in press) or are ready for publication. D. chapmani, D. improvisus and D. noricus remain unknown or insufficiently described to allow diagnostic differentiation from other species. Within the project, larvae were either identified by rearing eggs of previously identified egg-laying females in the laboratory or through molecular based associations of unknown larvae with field-collected adults. Associations are based on sequence data from the mitochondrial cytochrome oxidase I region (mtCOI).

Tab 1. Drusinae species so far reported in Austria, Germany and Switzerland (Lubini 2004, Malicky 1999, Robert 2001).

Anomalopterygella chauviniana (Stein)	Drusus mixtus (Pictet)
Cryptothrix nebulicola McLachlan	Drusus monticola McLachlan
Drusus alpinus (Meyer-Dür)	Drusus muelleri McLachlan
Drusus annulatus (Stephens)	Drusus nigrescens Meyer-Dür
Drusus biguttatus (Pictet)	Drusus noricus Malicky
Drusus chapmani McLachlan	Drusus trifidus McLachlan
Drusus chrysotus (Rambur)	Ecclisopteryx asterix Malicky
Drusus destitutus (Kolenati)	Ecclisopteryx dalecarlica Kolenati
Drusus discolor (Rambur)	Ecclisopteryx guttulata (Pictet)
Drusus franzi Schmid	Ecclisopteryx madida (McLachlan)
Drusus improvisus (McLachlan)	Metanoea flavipennis (Pictet)
Drusus melanchaetes McLachlan	Metanoea rhaetica Schmid

With respect to mouthpart anatomy, three distinct species groupings exist in Central European Drusinae: in Cryptothrix nebulicola, Drusus chrysotus, D. discolor and D. muelleri, mandibles with teeth around edges are present; this, together with additional setae on the legs, head-capsule modifications, and long filtering bristles on the first abdominal Sternum, identifies this group as carnivorous filterers (Figs. 1-6).

Figs. 1-6.

Fig. 1: Cryptothrix nebulicola, last instar larva, head

Fig. 2: Drusus discolor, last instar larva, head

Figs. 3-5 Drusus chrysotus, last instar larva. 3: right hindleg with filtering spines; 4: first abdominal sternum with filtering bristles; 5: head

Fig. 6: Drusus muelleri, head

In order to switch into filtering mode, the ventral section of the anterior case opening is most commonly attached to the substrate by silk. Then the legs are spread out, much as in Brachycentridae, and the filtering bristles exposed to the current as described by Bohle (1983) for Drusus discolor. The filtering bristles on the first abdominal sternum probably assist in catching prey drifting close to the stream bottom. According to gut analyses by Bohle (1983, 1987), aquatic insect prey in Cryptothrix nebulicola, Drusus chrysotus and D. discolor comprised larval Ephemeroptera, Plecoptera, Trichoptera and Diptera (Chironomidae and Simuliidae), as well as small fragments of Spermatophyta and Bryophyta. Only early instars of aquatic insects were consumed in toto, larger prey items were cut into pieces using the teeth on the mandible cutting edges.

In D.franzi and D. alpinus, mandibles with teeth around edges are present, but additional filtering bristles on legs and the first abdominal Sternum are lacking, which is typical for omnivorous generalists feeding on a wide spectrum of aquatic invertebrates, water mosses and aquatic plants (Figs. 11-13). In the remaining 15 species, a spoon-shaped mandible without teeth is present and additional setae and bristles are lacking, identifying this species group as scrapers feeding mainly on epilithic algae (Figs. 7-10). Generally, epilithic algal growth is much higher at lotic stream sections and midstream than in lenitic sections or near the banks (Gessner, 1955). This is why scraper Drusinae species are forced to expose themselves much more during feeding than omnivorous generalists feeding near the banks. In addition, in order to feed effectively, scraping Drusinae species do not fix their cases at the substrate as the filter-feeding Drusinae do. This results in a significant over-represention of scraper Drusinae species in the drift (up to 60 % of drifting caddis larvae) when compared with their relative abundance on the stream bed (40 %; Bacher & Waringer 1996).

Figs. 7-13.

Figs. 7-8: Drusus nigrescens, last instar larva. 7: head with spoon-shaped scraper mandible and pronotum with serrated ridge; 8: right midleg without filtering bristles

Figs. 9-10: Drusus melanchaetes, last instar larva; 9: head; 10: first abdominal sternum without filtering bristles

Figs. 11-13: Drusus franzi, last instar larva; 11: first abdominal Sternum without filtering bristles; 12,13: omnivorous Shredder mandibles with teeth along the anterior margin

The groupings observed in our genetic analyses reflect mouthpart morphology and feeding ecology. Three gene regions (mtCOI, mtLSU and nuWG) were amplified by means of PCR and then sequenced following the methodology described by Pauls (2004) and Pauls et al. in prep. For phylogenetic reconstruction, sequence data were analysed using a Bayesian Markov Chain Monte Carlo (B/MCMC) approach as implemented in MrBayes 3.1 (Ronquist & Huelsenbeck 2003). Groupings were significant for 1) epilithic grazers with smooth mandible edges, 2) carnivorous filterers with serrated mandible edges and filtering setae and bristles, and 3) omnivorous generalists with teeth on mandible edges. Phylogenetic hypotheses were tested using part of the B/MCMC tree sample. The frequency of trees agreeing with the null hypothesis was calculated by applying constraint-based tree filters in Paup* (Swofford et al., 1996) and equals the probability of the null hypothesis being correct (Ihlen & Ekman 2002, Lumbsch et al. 2004). Monophyly of genera Dusus and Ecclisopteryx was rejected (p <0.001). These genera are based on adult morphology. We conclude that in the Drusinae, larval morphology may be phylogenetically more informative than adult morphology and highly relevant for understanding the evolution of the group.