 Males of most species have a unique structure , a bristle @-@ bearing knob known as a " <unk> axillaris " , at the base of the rear wing . The abdomen in males is usually longer than in females and often has an extra lobe . The tip of the abdomen of females shows greater variation than that of males , depending perhaps on oviposition sites , and usually bears tufts of bristles for digging and a finger @-@ like extension . 
