The extraordinary array of mating systems in the Scolytidae and Platypodidae has been largely overlookrd by resrarchers interested in the evolution ofsexual behaviour. This paper proipides the first overview of reproducti\ e behaviour in this important and widespread group, known to most biologists only by the reputations of tree-killing taxa.Referred to generally as 'bark beetles', these insects chew egg tunnels inside a variety of (usually deadi plant tissues, though most species are either phloeophagous (breeding in the inner bark ofwoody p h t S J or xylomycetophagoris (all stages feeding on mutualistic fungi growing on sapwood or heartwood). In most species, permanent records of many aspects of reproductive behaviour are etched in the host; in many, engra\ings reveal female fecundity, eggs sired per male, hatching success, and offspring survivorship. Each gallery arm represents a good portion of a given female's lifetime reproduction. but in many species females commonly reemerge to reproduce in one or two additional sites.In most species of bark beetles, each female initiates her own gallery, to be joined later by a male. These monogynous gallery systems are associated with mating systems defined by how long males stay with females: in a few species, males seldom ife\,erjoin females under the bark; in the vast majority of species, males stay for part or all of the oviposition period then leave to seek other mates; and a few groups exhibit permanent monogamy, in that both sexes die in their only gallery system. \VliiIc these patterns emerge from an overview of the world scolytid fauna, the length ofmale residency has seldom been quantified, and the costs arid benefits associated with male mating strategies have not been measured for any bark beetle.Male-initiated monogyny is uncommon in Scolytidae, though the rule in Platypodidae; all instances of which I am aware are summarized from a phylogenetic perspective.Inbreeding polygyny with highly biased sex ratios has arisen at least seven times in Scolytidae. These taxa are usually characterized by males being dwarfed, flightless, and uncommon. Sex determination is known for only a few examples, hut both haplodiploidy and diplodiploidv have been reported.Multiple origins of harem poly~qny (otherwise rare in invertebrates] add an exciting dimension to the comparative and experimental study ofscolytid mating systems. In harem polygynous taxa, males initiate gallery construction. I summarize what little can be learned from the literature about the fine structiire of harem polygynous mating systems in bark beetles, and the problem of measuring reproducti1.e success. Data on the nature of harem polygyny in Pityophthorus lautus are presented, illustrating ( a , the fluidity ofharems; (b) that average eggs laid per gallery arm is relatively unaffected by harem size, but strongly influenced by resource quality; (cl that male egg-gain is strongly correlated with territory quality (a consequence of (b) above); and (di the temporal patterning of immigration and emigration ...

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Males of aculeate Hymenoptera differ in the behavioural adaptations employed to locate and secure mates. The ecological and evolutionary bases of these differences are explored in this paper. Male bees and wasps search for females by patrolling widely within emergence-nesting areas or within patches of flowers attractive to conspecific females, or by waiting at landmarks, at specific emergence sites, or at nests. Nest dispersion, flower distribution, the type of female mating system and the nature of male-male competition appear to be key factors in determining the mate-locating behaviour of'males. Of special interest in multiple-mating by females, which may be an evolutionary response to the costs of attempting to resist copulation in certain situations. When polyandry occurs, males are under selection pressure to be the last male to copulate with a female prior to oviposition if sperm precedence occurs. In species in which females mate just once, a selective premium is placed on being the first male to reach a virgin female. In either case, because receptive females are a limited resource, there is intense competition among males for access to the resource. The density of competitor males may play an important role in determining whether holding a relatively restricted territory is preferable to the strategy of patrolling widely at various sites which may have females. Territoriality is practiced by males of several species of aculeate Hymenoptera when the number of male competitors is relatively few in number and the distribution of emergence sites or foraging areas of females is clumped in space.

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