With the use of an exit just larger than the size of most flies,
tabanids typically enter the collecting chamber by crawling
through, rather than flying through. Most Tabanus horse
flies spend only a minute or two flying about before leaving
via the top exit. Some Hybomitra horse flies accumulate
inside the body of the trap when catches are high. Horse flies in general fly repeatedly into
the funnel before crawling up and out. They fly through an exit hole only when it is very
large. Deer flies behave the same way but spend much more time simply resting inside the
cone. They also tend to focus their activity on corners and edges instead of focusing on the
apex of the cone. Stable flies take several minutes to exit as they only gradually work their
way to the apex. Mosquitoes spend the most time of all flies resting inside the cone; many
likely die and fall onto the ground without being counted when traps are checked in the
evening. Some black flies also spend most of their time crawling inside the cone and may
not be counted properly if one is not checking traps regularly.
Once in the top collector, flies fly and crawl about on all surfaces and can potentially
escape. The use of a long, transparent netting sleeve or some other diversion system
concentrates activity away from the point of return. Although not intended for holding
such large numbers, I have caught up to about 2,500 tabanids in a sleeve collector over just
a few hours