BODY TEMPERATURE AND FLIGHT MUSCLE RATIO IN THE
BURROWING MAYFLY HEXAGENIA BILINEATA

Joseph R. Coelho
Department of Biological Sciences, Western Illinois University, Macomb, IL 
61455

Flight and thermoregulatory ability in aquatic insects varies from exceptionally
high (Odonata) to extremely limited (Plecoptera), but flight physiology is poorly
understood in the Ephemeroptera. Since Hexagenia is the largest-bodied North
American genus, it provides a convenient model for the study of mayfly
thermoregulation and flight.  The thorax temperature (Tth) of free-flying H.
bilineata (21.0  0.07 (36) C) was significantly different (t-test, P < 0.0001) from
ambient temperature (Ta, 20.0  0.06 (36) C), and the gradient increased with
decreasing Ta,  providing  a hint of thermoregulatory ability. However, H.
bilineata is clearly ectothermic, and the gradient between Tth and Ta is probably
not biologically significant, as one degree provides little increment in biochemical
or physiological performance. The relatively large wings (101.8  4.0 (6) mm2 for
one ipsilateral pair) of H. bilineata give it a relatively low wing-loading (1.705 
0.046 (6) N/m2), about one tenth that of honeybees.   The known correlation of
Tth with wing-loading may explain the low Tth of Hexagenia.  Endothermy is
expected to evolve in large, flying insects, but ecological considerations make
ectothermy a logical alternative for mayflies. On Pool 19 H. bilineata emerges
within the first three weeks of July, which typically has warm and stable weather. 
Being nocturnal prevents mayflies from benefitting from solar radiation, but also
buffers them from its variation. Therefore, H. bilineata seldom faces cold stress or
heat stress.  As endothermy is energetically very expensive, ectothermy is sensible
for an animal that lives for only one day.  High thorax mass (0.020  0.0010 (14)
mg) relative to body mass (0.035  0.0014 (14) mg) resulted in very high ratio of
flight muscle mass to body mass (flight muscle ratio or FMR, 0.556 
0.0096(14)).   FMR is directly related to force production during flight.  Hence,
high FMR is expected to result from selection pressure on maneuverability and
load carriage.  The mean FMR of H. bilineata is much higher than that of most
bees and wasps that have been examined, although all such Hymenoptera would
almost certainly be classified as better fliers than H. bilineata.  The high mayfly
FMR may be a result of freedom from constraints, rather than selection on flight
ability.  Primarily, mayflies do not feed as adults; therefore, they have no digestive
tract, and only rudimentary mouthparts.  Mayflies also have small, fragile legs.
These considerations alone (gut, legs) may make up the difference.  Although one
might expect an insect with high FMR to be strongly endothermic, the
relationship between FMR and endothermic ability has not been established.

Keywords: ectothermy, flight, mayfly, insect, wing-loading

Presenting author:  Joseph R. Coelho
Department of Biological Sciences, Western Illinois University, Macomb, IL 
61455
Phone: 309-298-2521; FAX: 309-298-2270; Email: JR-Coelho@wiu.edu
Preference for poster presentation, little willingness to convert to platform
presentation.