The guarding posture of females in the subsocial bug Elasmucha dorsalis ( Heteroptera : Acanthosomatidae )

The subsocial bug Elasmucha dorsalis lays egg masses on the underside o f the leaves o f female plants o f Aruncus dioicus. Each bug straddles her eggs and shields her offspring with her body, until they moult to the 2nd instar. Females that attended aggre­ gations o f 2nd and later instar nymphs feeding on fruit o f inflorescences often settled close to the basal part o f or just below the ag­ gregation, and faced towards the base o f the inflorescence. The nymphal aggregations often seemed to be too large for females to guard effectively. The position and orientation o f females attending 2nd or later instar nymphs probably enabled them to detect predators walking towards the nymphs.


INTRODUCTION
The offspring of many subsocial insects are potentially vulnerable to predators and/or parasitoids, but are effectively protected against these enemies by their mother (or father in some species) (Tallamy & Wood, 1986;Trumbo, 1996).In the subsocial Elasmucha bugs, a variety of natural enemies attack the immatures (e.g., Melber et al., 1980).
The defensive tactics of a parent, and their offspring's behaviour, will be selected for increased efficiency against the enemies of the offspring in the arms race between subsocial insects and their enemies.Thus, the effectiveness of parental defence may depend on the enemy species, each of which has different attack tactics.There are large differences in the effectiveness of parental defence of subsocial phytophagous insects against different enemies, predators and parasitoids (e.g., Eberhard, 1975).Such differences may be associated with the position and posture of the parent attending the offspring (Kudo, 1996;Kudo & Ishibashi, 1996).
In this paper, the guarding position and posture of the mother of the subsocial bug Elasmucha dorsalis (Jakovlev), which possibly serves to guard against walking predators, but not parasitoids, is described.

RESULTS AND DISCUSSION
Females invariably straddled their offspring from the egg to the second instar (76 out of 76 females with eggs, 42 out of 42 females with 1st instar nymphs, and 9 out o f 9 females with newly moulted 2nd instar nymphs).The same was observed in this population in 1988 (Kudo et al., 1989).
When attending later instar broods on inflorescences, how ever, females changed their position (Table 1; x2 = 114.7,d.f.= 6, P < 0.001, cells of the developmental stage with small ex pected values were combined): they often settled close to the base of a nymphal aggregation or just below the aggregation.Most of these females faced towards the base of the inflores cence (Table 1).
Second or later instar nymphs were usually found on inflores cences.Two female-attended aggregations, in which the nymphs were moulting to the 3rd instar, were found on the un derside of leaves.One female straddled the nymphal aggrega tion and the other had settled on the underside of the petiole of

Field
E. dorsalis females were marked soon after oviposition and censused periodically.The brood condition, which was that of the majority of individuals in each brood, was catego rized into one of 12 classes: with one of 6 categories for the de velopmental stage, eggs to the fifth instar, and two for the feed ing site, inflorescence or leaf.The position and orientation of fe males were recorded at the time when they first appeared with the brood of a particular developmental stage (thus, females were counted only once for each class).Females adopted one of three positions: (1) settled centrally (i.e., half way between the most basal and apical individuals in an aggregation of offspring or straddling the aggregation), (2) settled in a basal position (i.e., within one-fourth of the length of an aggregation of offspring from the base or just below the aggregation), (3) settled in an apical position (i.e., within one-fourth of the length of an aggre gation of offspring from the apex or just apical to the aggrega tion).Whether females orientated towards the apex, the base or sides, was also recorded for females that settled on an inflores cence.Additional data were also obtained from the research conducted on the same population in 1989.In 1990 and 1991, the size of aggregations (the distance be tween the most basal and apical individuals in an aggregation) on inflorescences was measured.The number of nymphs per ag gregation was not determined, because counting disturbed the nymphs.
Fig. 1.Frequency distribution of the size of nymphal aggrega tions on inflorescences.Aggregation size: the distance between the most basal and apical individuals in an aggregation.

Table 1 . The posture adopted by Elasmucha dorsalis females attending nymphs on inflorescences of Aruncus
dioicus.