Collembola) in Relation to Predation

ANTENNAL DAMAGE AND REGENERATION IN SPRINGTAILS (COLLEMBOLA) IN RELATION TO PREDATION

G. ERNSTING1 and D. S. FOKKEMA2 (1: Department of Biology, Free University, de Boelelaan 1087, 1081 HV Amsterdam, The Netherlands) (2: Zoological Laboratory, State University of Groningen, Kerklaan 30, 9750 AA Haren [Gr.], The Netherlands)

SUMMARY

(1) A significant proportion of unsuccessful attacks by the predator Notiophilusbigut- tatus (Carabidae) on Orchesellacincta (Collembola) results in damage to the springtails' antennae. (2) Regeneration of the loss occurs at several moults following the amputation, and is attended with an increase in moulting frequency. (3) The regenerated antenna is shorter and always less segmented than normal. (4) The frequency of O. cincta with damaged or regenerated antennae in the field depends on the size (i.e. age) of the springtails. (5) Use of these frequencies in evaluating the role of predators in the dynamics of the springtail population is discussed.

INTRODUCTION

In the surface dwelling springtail species Orchesella cincta L. predation accounts for a large proportion of total mortality (ERNSTING & JoossE, 1974). One of the most important predators is Notiophilus biguttatus F., a visually-hunting carabid beetle. From laboratory studies on predation, using these two species, it appeared that many attacks of the beetle are unsuccessful (ERNSTING & JANSEN, 1978; ERNSTING & MULDER, 1981). However, a significant proportion of these unsuccessful attacks inflict damage on the prey. In most cases the damage is sublethal and concerns the loss of a leg or, more frequently, the loss of one or both antennae or part of them. Throughout their lives springtails, being ametabolous insects, moult frequently which renders them the ability to regenerate these appendages. For 0. cincta regeneration of the antennae following amputation was already described by LUBBOCK (1873); he also noticed the high frequency of antennal mutilations, consisting of reduced segmentation, to be found 477 in the genera Orchesella and Tomocerus. In the field 0. cincta specimens with damaged or abnormal segmented antennae are indeed frequent. These field and laboratory observations lead to the assumption that unsuccessful attacks of predators are the main cause for damage to the antennae. If so, one might consider using the percentage of damaged or regenerated antennae as an index for instantaneous respectively cumulative predation pressure, as has been done in a variety of studies on other prey (listed by VERMEY, 1982). Obviously, direct observations of predation are to be preferred, but much harder to obtain. The present paper presents data to sustain the above mentioned assumption and data to explore the possibility of using the frequencies of damaged and regenerated antennae as indices for predation.

MATERIALS AND METHODS Animals

Notiophilusbiguttatus is a rather small (5-6 mm) diurnally active predator. It is a common species of woodlands where it prefers relatively open and dry places. The beetle feeds mainly on springtails, which are captured after visual detection. Visual detection and attacks only occur when the prey (or part of it) moves. Orchesellacincta (Collembola) is a common surface dwelling springtail species, which is relatively drought-tolerant 000SSE & GROEN,1970; VERHOEF& WITTEVEEN,1981). This species, due to its abundance, locomotory activity and coincidence in space with N. biguttatus,is an important prey of the latter. The antennae of this species, like those of other hemiedaphic springtail species, are relatively long: in adult specimens their length is over half the length of their body. The antennae in springtails consist primari- ly of 4 segments. However, in the genus Orchesella,due to secondary annulation, the antennae are 6-segmented (GISIN, 1960). Moulting occurs throughout the springtails life, because it is not only related to growth, but it is also a means to get rid of waste products (HUMBERT,1977; JOOSSE& BUCKER, 1979). According t0 JOOSSE& VELTKAMP (1970) moulting in 0. cincta, as deduced from the number of exuviae found in a mass culture, occurs every 6 days at 20°C. The springtails used in the two experiments ranged in length from 2 to 3 mm.

Experiments In the first laboratory experiment 96 attacks of N. biguttatuswere observed. For that purpose 46 Q-beetles, individually kept in open glass jars, were confronted with 3 prey specimens each. The beetles had been starved for 24-30 hours. The attacks of each beetle were observed until a prey was captured. For each attack the result was noted (success, complete failure or failure leaving antennal damaga) and also if it was directed towards the head region or the body. Apart from these observations those prey with antennal damage were kept for 7 weeks individually in small jars for regeneration. In the secondlaboratory experiment 3 segments of both antennae of 50 0. cincta specimens were amputated. Amputation occurred during anaesthesia with carbon dioxyde; the control animals (50) were also subjected to anaesthesia. Then, twice a week for 5 weeks, the number of exuviae of both groups were counted and the length of the treated antennae was measured. During this period the animals were kept in groups of 5 in small jars.