A.G Wells 
Right: This female has emerged from 
her burrow in the ground, climbed a 
piece of vegetation, and is calling 
for a male by releasing into the air 
a sex pheromone unique to her 
species. Far right: Diverse methods 
of feeding females are used by the 
male wasps. In this species, 
the male regurgitates a drop of nectar 
on a leaf and the female, in copulo, 
twists her body underneath the male 
to imbibe the food. 
are species in which a male may help his 
partner dig a burrow nest and provision 
the brood cells with choice dung balls on 
which the female lays her eggs. In still 
other insects, males collect food that 
their mates consume. For example, in 
certain empidid flies and the unrelated 
hanging flies, the male captures an in- 
sect prey, kills it, and then offers the 
food to a female as a nuptial gift that she 
consumes during copulation. 
This type of behavior poses an evolu- 
tionary puzzle: Why are these males 
able to transmit more of their genes than 
would potential competitors using all 
their time and energy to reach a larger 
number of receptive females? Randy 
Thornhill of the University of New 
Mexico has provided a plausible expla- 
nation for the genetic success of helpful 
male hanging flies. He has found that 
females that do not receive a nuptial 
present simply refuse to copulate. More- 
over, if the male offers a gift that is not 
sufficiently large, the female will termi- 
nate the copulation prematurely and 
seek out another male. Once, the female 
has fed to satiation on a large nuptial 
gift and copulated with the male that 
presented it, however, she becomes un- 
receptive for a time to further matings 
with other males. During that interval, 
she lays a series of eggs fertilized by her 
recent mate’s sperm. The male’s gift to 
the femal? therefore advances his own 
genetic success. 
A similar explanation may apply to 
the evolution of the male mating behav- 
ior displayed by some thynnine wasps of 
Australia. There are well over 500 spe- 
cies of these wasps in Australia, many of 
them undescribed and most of them 
unstudied. Judging from the results of 
the few behavioral studies conducted 
thus far, however, some features of their 
biology may be widespread among the 
group. To begin, the females of these 
supposedly primitive wasps are wing- 
less, a trait that is related to their 
method of molelike burrowing through 
moist soil in woodlands and meadows in 
search of beetle larvae. When a tunnel- 
ing female encounters an appropriate 
victim, she stings and paralyzes the lar- 
va, lays an egg on the prey, and exca- 
vates a small chamber around it. The egg 
she leaves behind hatches into a grub that 
consumes the beetle larva and eventually 
metamorphoses into an adult wasp. 
In a number of species, after a newly 
adult female emerges from her pupal 
case she burrows to the surface and 
crawls up a grass stem or weed. When 
she has reached a point well above the 
ground, she adopts the calling posture 
standard for her species. In many cases 
this involves perching head-up, with the 
head held out from the stem. But in 
other species, the female turns to face 
down the stem or twig. And in still 
others, the female holds her abdomen 
stiffly out at an angle from the perch. In 
all species studied to date, the female 
releases a sex pheromone while in the 
calling position. 
Male thynnine wasps possess wings, 
and often their major activity is to fly 
about in areas in which females are 
calling. In my studies of several species 
in eastern Australia, I captured and 
then marked males with dots of paint on 
the back of the thorax. After releasing 
the marked wasps, I later attempted to 
net patrolling males in the same area. 
Individuals were regularly caught as 
much as two weeks after they had been 
marked, indicating that the males had 
home ranges in which they searched day 
after day for mates. 
When a male detects the pheromone 
of a female, he zigzags upwind toward 
her. (Although females of several spe- 
cies may release pheromones in the same 
area, they only attract males of their 
own kind, presumably because each spe- 
cies produces a unique sex pheromone.) 
When a male reaches the grass blade or 
plant stem on which the female is perch- 
ing, he frantically half-scrambles, half- 
flies through the foliage in an effort to 
reach the female before his competitors 
can find her. The male that makes first 
contact almost always wins the female, 
quickly picking her up and carrying her 
off in copulo in a nuptial flight. In some 
thynnine species, the male transports 
the female some distance from the call- 
38 
