Factors affecting microdistribution of two species of burrowing dragonfly larvae, with notes on their biology (Anisoptera: Gomphidae)

Progomphus obscurus and Gomphus externus larvae were studied in both laboratory and field during July, 1976, regarding their burrowing habits and their association with substrates. They were found to burrow completely beneath bottom substrates by using their front and middle legs, never deeper than 2 cm; small larvae burrowed less deeply. — Field studies revealed that both spp, occurred in the stream in relatively high numbers and were distributed in either a contagious or random manner. Population structure of both spp. was similar and consisted mainly of late instars. Statistical analysis of physical factors measured (substrate particle size, digested organics and detritus) and the occurrence of each sp. demonstrated a correlation between particle size and occurrence of each sp. P. obscurus lends to inhabit microhabitats in stream where sand substrate (1-0.625 mm) predominates. Microdistribution of G. externus is most strongly correlated with the amount of silt/clay substrate present (less than or equal to 0.0625 mm), but percentage of organics was also significant. Although other environmental factors may have had an effect on the distribution of these 2 spp., the primary factor was substrate particle size.