1975] 
Robey — Pachydiplax longipennis 
95 
Interspecific Behavior 
Pachydiplax longipennis males are unusually aggressive dragon- 
flies when compared with other Libellulids. They react with par- 
ticular ferocity to three larger sympatric species whose pruinosity 
color is similar to the grey-blue color of Pachydiplax longipennis. 
The first, Libellula cyanae Fabricius, most common at Pickman 
and the Five Fields ponds has a slightly darker pruinescence, cover- 
ing the entire adult male body. Libellula incesta , common only at 
the Shade Street pond, has a. much darker, almost black, pruinescence. 
The third, Erythemis simplicicollis Say, has the same color as Pachy- 
diplax longipennis , covering a teneral green on the dorsal surface of 
the abdomen and synthorax. It occurred at all three of these ponds. 
Libellula incesta and L. cyanae sometimes occupy perches and enter 
the territory of Pachydiplax longipennis. Erythemis simplicicollis 
usually preferred to perch on an exposed log or rock, situations rarely 
used by P. longipennis. Despite its smaller size, P. longipennis would 
display and chase all three of these species that entered its territory. 
Other dragonflies commonly present at these ponds included : 
Plathemis lydia, Sympetrum rubicundulum , Leiicorrhina intacta, 
Gomphus furcifer , Perithemis tenera, Libellula pulchella L. luctosa 
and Ladonia julia. P. longipennis males generally ignore them, pre- 
sumably because of their different appearance. 
Coloration 
Coloration patterns of recently dead males (killed by chilling) 
were compared in visible and ultraviolet light. It was found that the 
blue pruinose areas on the abdomen though dull under visible light 
have marked reflectivity in the ultraviolet. Highly reflective prui- 
nescent areas were also found in males of Libellula cyanae , L. 
incesta and Erythemis simplicicollis (see Fig. 2), but were absent 
in females of these species 
Some odonates, including a libellulid, have been shown by physio- 
logical methods to be UV-sensitive (Goldsmith and Bernard, 1974: 
Table 5). Why is it that these, more stationary, libellulids have 
pruinescence in similar areas while in others it is a different color, 
located in different areas or absent all together? Species such as 
Libellula pulchella have only white pruinescent spots on their wings 
and are much more mobile than P. longipennis. In light of these 
preliminary findings, the possible role of ultraviolet patterns in odo- 
nate behavior deserves careful attention. 
