TABANTD EGG PARASITE PHANURUS EMERSONT GIRAULT 6 



The eggs of Tabanus hyalinipennis are deposited mostly on stones 

 from 3 to 15 inches in diameter, projecting above the water from 

 1 to 12 inches in rapids where the streams spread over the gravel 

 beds in sunny, open places. (Fig. 1.) Some eggs are deposited on 

 plants growing in the rapids and along the water's edge. The eggs 

 have never been found in the swamps and rarely ever in places 

 shaded for most of the day. The parasite prefers the open, sunny 

 stretches of the streams, and the eggs on stones are usually most 

 heavily parasitized, but at times the eggs on the plants are quite 

 as heavily parasitized. The adults are most active during bright 

 hot weather and have never been observed active in misty or rainy 

 weather. The heaviest infestations of the horseflies have occurred 

 following seasons when there were heavy and continued summer 

 rains; this may be accounted for by the facts that during such 



Fig. 1. — Gravel bar along- the Frio River, where Phanurus emersoni parasitizes a 

 large percentage of Tabanus eggs 



seasons the fly-breeding grounds in the streams are greatly increased 

 and the activity and breeding of the parasite are retarded. 



LIFE CYCLE AND SEASONAL ACTIVITIES OF THE PARASITE 



The adults copulate almost as soon as they emerge and are dry. 

 Oviposition begins in from 4 to 24 hours after emergence. In the 

 act of ovipositing, the female inserts the ovipositor into the mem- 

 brane of the fly egg. She appears to be able to insert the ovipositor 

 between the eggs of the top layers into the eggs below. The females 

 prefer fresh eggs to older ones and have never been induced to 

 oviposit in eggs more than 5 or 6 hours old. The average rate 

 of oviposition in fresh eggs has been observed to be one every 25 

 seconds, but it takes longer to oviposit in older eggs. From 1 to 8 



