ONE SIZE FITS ALL - RELATIONSHIPS BETWEEN THE SIZE AND DEGREE OF VARIATION IN GENITALIA AND OTHER BODY PARTS IN 20 SPECIES OF INSECTS AND SPIDERS

Hypotheses regarding the function of elaborate male genitalia were tes ted in a sample of insects and spiders by comparing their allometric v alues (slopes in log-log regressions on indicators of body size) with those of other body parts. Male genitalia consistently had lower slope s than other body parts. Perhaps as a consequence of this pattern, gen italic size also tended, though less consistently, to have lower coeff icients of variation than did the size of other body parts. The morpho logical details of coupling between males arid females in several spec ies clearly indicated that selection favoring mechanical fit is not re sponsible for these trends. Sexual selection on male courtship structu res that are brought into contact with females in precise ways may fav or relatively low allometric values, in contrast to the high values se en in the other sexually selected characters (usually visual display d evices) that have been studied previously, because a female's own size will influence her perception of the contact courtship devices of a m ale.