Territoriality and fighting in a colonial thrips, Hoplothrips pedicularius , and sexual dimorphism in Thysanoptera

.1 Hoplothrips pedicularius (Haliday) (Thysanoptera: Phlaeothripidae), a tubuliferan thrips in which males possess greatly enlarged forelegs, lives in colonies on Stereum fungus. 2 Females oviposit onto communal egg masses, and males fight by grasping and stabbing with their forelegs in territorial defence of oviposition areas. Prolonged escalated fights occur between males who are of similar size. 3 Larger males usually win fights and become dominant at the oviposition area. Dominant males secure 80% of matings, and mate most frequently during oviposition periods, with an ovipositing female. 4 Smaller, subordinate males avoid fights and attempt to 'sneak’copulations. However, they occasionally challenge the dominant male. Challenges tend to follow copulations by the subordinate male and occur more frequently between males who are of similar size. 5 Subordinate males who eventually leave the oviposition area are larger than those who remain, have frequently challenged the dominant male, and have more frequently been stabbed. 6 Sexual dimorphism in thrips is associated with gregariousness, claustral habitats, female‐biased sex ratios, and male winglessness. In thrips genera in which males exhibit foreleg armature, males are larger relative to females. The ecological circumstances promoting sexual dimorphism and male fighting in spatially‐structured populations are discussed.