FLIGHT ARCHITECTURE DETERMINED BY PHYSICAL CONSTRAINTS OR BY NATURAL-SELECTION - THE CASE OF THE MIDGE CHIRONOMUS-PLUMOSUS

Males of the midge Chironomus plumosus fly solely to mate. They mainta in station for long periods in moving air. Females patrol in search of males and, after receiving a spermatophore, By to oviposition sites. The requirements of flight in males and females are therefore fundamen tally different. Females are larger than males, on average, so these d ifferences could stem from scaling rules governing the geometry of spa ce. The same explanation might apply to Right differences within the s exes and even to peculiarities of flight architecture in C. plumosus c ompared to other flying animals. In other words, might Right design be accounted for entirely by mechanical constraints without recourse to natural selection? To test this hypothesis, the power output of C. plu mosus was measured as size-specific muscle mass. Contrary to expectati on, little evidence was found of scaling effects in this measure of po wer. Despite its being among the smallest of animals to fly, C. plumos us turns out to have the largest mass of flight muscle, relative to bo dy size, yet found among animals. Differences both between C. plumosus and other species and within C. plumosus are, in general, more readil y accounted for by the requirements of the mating system within the vi scous universe encountered by small flying animals. We conclude that i t is natural selection rather than mechanical constraint that is the p rimary influence determining the architecture of flight in this small animal.