Adults of these two species of bee flies both emerge in August, coinciding with the blooming of rabbitbrush, Chrysothamnus nauseosus, upon which adult flies of both sexes exclusively feed (females eat nectar and pollen, males eat only nectar).
Lordotus miscellus
A
lone male L. miscellus is in the tiny box at the bottom of this
picture; he is defending the rabbitbrush shrub behind him, giving you an
idea of scale. Males of L. miscellus defend this food source
against other males looking for mates, and simultaneously they scan for
females approaching the shrub to feed. Superficially, this system
appears to be classic resource-defense territoriality, but a closer look
suggests that males of L. miscellus use the rabbitbrush shrub as
sort of a "peacock tail", attracting females ready to mate. Males defend
only the largest shrubs in the population, whereas females visit shrubs
of all sizes to feed, dependent only on whether the shrub has open flowers.
Females mate only once, immediately upon emergence and before they begin
to feed. I hypothesize that females seek out the largest rabbitbrush
individuals, precisely to find males to mate with; the largest rabbitbrush
are highly conspicuous when in full bloom and stand out for quite a distance. 
This
"beacon" allows females to save time in detection of mates and also to
find the most fit males, which can defend this highly valuable resources
against all intruders. Males engaged in territorial "sallying" behavior
are the larger males in the population; smaller "sattelite" males hang
around the large rabbitbrush, presumably waiting for opportunities to mate.
Females of L. miscellus are capable of rejecting males with which
they do not wish to mate; they shake them off with a rolling motion similar
to a dog shaking a wet coat. This research has not yet been published
because the population of L. miscellus crashed during the drought
of the late 80's, and is only now recovering to allow adequate sample sizes
for testing these hypotheses. This project is available for a Master's
student.
Lordotus pulchrissimus
The life histories of male and female
L. pulchrissimus also center
on
the distribution and blooming of rabbitbrush,
Chrysothamnus nauseosus.
Here is a male of L. pulchrissimus that I have interrupted during
the morning's territorial activity to mark him (see the pale spots of orange
and green on his thorax; these are two of the three color dots of his unique
mark). Males engage in true lek territoriality, gathering each morning
at traditional locations. 
Early
in the cool morning, males warm on the sand below the aerial swarm, and
when a sufficient body temperature is achieved for strenuous hovering flight
and chases, the male returns to the swarm. The air circulation generated
by the rapidly beating wings cools the male, more rapidly in the cooler
morning temperatures than later in mid-morning. Therefore males land frequently
to warm between 10 am and 10:30 am on a typical August morning. By 11 am,
temperatures have risen sufficiently to allow males to remain continuously
in the air. At this time, all males present at a lek site are engaged in
territorial activity, i.e., frequent attacks and chases in the swarm, perhaps
as males vie for some sort of optimal position in the swarm. Not surprisingly,
females ready to mate arrive at the lek sites then, with the majority of
copulations occurring in the half hour around 11 am. Females mate only
once, as soon as 
they
emerge. We can age the females because the color of their "fur" fades in
the sun. Newly emerged females are invariably this deep reddish-orange
color; older females fade to yellow and then white.
The life expectancy of adult males is about 5 or 6 days; the life expectancy
of adult females is about 12-14 days. Males burn themselves out quickly
from the exertion of fighting in the aerial swarm. Not only does this require
a great deal of energy (lek activity stops by noon; males rest until the
next morning, when they spend about an hour feeding on nectar before heading
to the lek sites) but also males batter each other with recurved spines
on the costal vien (leading edge) of the wings. Females spend the rest
of their adult lives feeding on nectar and pollen and basking on the sand
in the cool mornings and late afternoons. Apparently they are developing
the large eggs that each hatch into a large, vigorous planidium larvae,
which then burrows actively in the sand until it finds a host.
The sites where males gather each morning of their adult lives (hereafter
lek sites) are in the same places year after year, although all males of
each generation die by early September. 
This
figure shows the original 9 lek sites in a 400 m by 700 m mapped area of
late Holocene, semi-stablized sand dunes, dominated by the perennial shrubs
species rabbitbrush (Chrysothamnus nauseosus) and greasewood (Sarcobatus
vermiculatus) with an understory of grasses and herbaceous species.
Lek sites are on average 120 m apart, and the landing diameter of each
lek site is an average of 15 m. Some leks always have many males; some
always have few, suggesting that some lek sites are (for whatever reason)
more favorable for encountering mates. Most lek sites have not moved positions
in the nearly 20 years that we have been censusing them, with one interesting
exception of a lek located on top of a dune ridge. Most lek sites are on
mid-dune slopes or swales. One of the sites with a small lek has not had
males present for over 5 years now; I assume that this lek site has "winked"
out. While no new lek sites have appeared in this study area, we do see
lone males engaging in aerial swarm behavior here or there in odd places,
suggesting that there is always a potential for a new lek site to form.
Nonetheless, the persistence of these traditional sites is impressive.
What causes the flies to be attracted to the same location, with such precision?
The main surviving hypothesis is that males deposit a persistent pheromone
(high molecular weight) on the sand when they warm. We investigated the
leading alternative hypotheses, which consist of males being attracted
to some kind of environmental swarm marker convention, but the data show
no evidence of any such environmental marker. Leks sites are randomly distributed
in suitable habitat. In fact, our efforts to find such an environmental
marker by measuring all conceivable environmental variables that might
attract flies resulted in a large study of the vegetation and physical
geography of the sand-dune ecosystem itself.
This work was funded by NSF-FAW GER 90-23789 to Catherine Toft.
Publications:
-
Toft, C.A. 1984. Activity budgets in two species of bee flies (Lordotus: Bombyliidae, Diptera): A comparison of species and sexes. Behavioral Ecology and Sociobiology 14:287-296. PDF
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Toft, C.A. 1989. Population structure and mating system of a desert bee fly (Lordotus pulchrissimus: Diptera, Bombyliidae). I. Male demography and interactions. Oikos 54:345-358. PDF
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Toft, C.A. 1989. Population structure and mating system of a desert bee fly (Lordotus pulchrissimus: Diptera, Bombyliidae). II. Female demography, copulations and characteristics of swarm sites. Oikos 54:359-369. PDF
Related studies:
-
Toft, C.A. 1987. Activity budgets and habitat use in two coexisting species of sand wasps (Microbembex: Sphecidae, Hymenoptera). Ecological Entomology 12:427-438. PDF
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Toft, C.A. 1987. Population structure and survival in a solitary wasp (Microbembex cubana: Hymenoptera, Nyssoninae). Oecologia 73:338-350. PDF
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Fraizer, T. 1997. A dynamic model of mating behavior in digger wasps: the energetics of male-male competition mimic size-dependent thermal constraints. Behavioral Ecology and Sociobiology 41:423-434.
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Fraizer, T.D. 1996. Sexual size dimorphism: mating systems and sexual selection for large males in the digger wasp Microbembex (Hymenoptera: Sphecidae). Ph.D. Thesis, University of California, Davis.
Public service publications (not peer reviewed):
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Toft, C.A. 1987. Population biology of bee flies. (I). The Mono Lake Newsletter 9(4):11-12.
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Toft, C.A. 1988. Population biology of bee flies. (II). The Mono Lake Newsletter 10(3):11-12.
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Toft, C.A. 1989. Population biology of bee flies (III). The Mono Lake Newsletter 11(3):12.
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