PAN-PACIFIC ENTOMOLOGIST 
Vol. 55, No. 1, p. 73 
SCIENTIFIC NOTE 
AN EXTRA MIDTIBIAL SPUR IN AN ISLAND POPULATION OF 
TRYPOXYLON TRIDENTATUM PACKARD 
(HYMENOPTERA: SPHECIDAE: LARRINAE: TRYPOXYLONINI) 
One apical midtibial spur (Fig. 1) is characteristic of all sphecid wasps in 
the subfamily Larrinae (Bohart and Menke, 1976, Sphecid Wasps of the 
World, Univ. Calif. Press, Berkeley, 695 pp.). Therefore, I was surprised 
to find that a second spur is often present on Trypoxylon tridentatum tri- 
dentatum Packard specimens from Santa Cruz Island, Santa Barbara Co., 
California. One hundred and two specimens in the collection at the Uni¬ 
versity of California at Davis were reared from 37 trap stems by D. S. 
Horning. The trap stems, all from Canyon del Medio on the island, were 
collected in September 1968. In 25 of 50 specimens reared from 14 stems, 
there was a second midtibial spur; specimens reared from the remaining 23 
stems were normal. The second spur is variable (Figs. 2 and 3) even on 
Figs. 1-3. Posterior view of left midtibia of Trypoxylon tridentatum : 1, normal from Santa 
Ynez Mts., Santa Barbara Co., Cal. (mainland); 2, female, and 3, male, with additional midtibial 
spur—both are from Canyon del Medio, Santa Cruz Isl., Cal. 
different sides of the same specimen. Eight specimens had the additional 
spur on only one side. The presence of the abnormal additional spur in a 
population on a small, isolated island suggests that genes controlling expres¬ 
sion of the spur are perhaps maintained in the gene pool by unusual selective 
pressure or genetic drift. 
Rollin E. Coville, Division of Entomol. and Parasitol. Univ. Calif, Berke¬ 
ley, CA 94720. 
