302 
Psvche 
[September-December 
The mesothoraeic leg of Gerris as support and oar 
The modification of the coxal and trochanteral orientation 
of the gerrid mesothoraeic leg has brought about a horizontal 
and lateral positioning of the leg which creates a wide base of 
support for the animal on the surface of the water. This orienta- 
tion of the leg also rests the distal tip of the tibia and the whole 
of the tarsus on the surface of the water and puts the leg into a 
good position for the rowing stroke. If the leg had a more verti- 
cal orientation, only the tip of the tarsus would be on the surface 
of the water, providing a much smaller pushing area during the 
rowing stroke. During this stroke the leg sweeps powerfully back- 
wards, pushing against the meniscus of the water surface (Darn- 
hofer-Demar, 1968); such a stroke would be impossible if the 
leg were orientated vertically. Moreover, the greater the area 
of the leg resting on the water surface, the more efficient leg re- 
traction will be in rowing. Gelastocoris walks on land and so a 
vertical orientation of the leg is a necessity. Befostoma swims 
underwater so that any orientation of the leg would push aside an 
equal volume of water. In Gerris the modification of the muscles’ 
function follows directly from the change in orientation of the 
leg, and the increase in the size of muscles 46 and 47 leads to a 
very powerful rowing stroke so that, with a single stroke, the 
animal can cover a distance several times its own length (Darn- 
hofer-Demar, 1968). 
Acknowledgments 
I should like to thank Drs. I. B. Sprague and M. C. Parsons 
for reading early drafts of his manuscript. This work was sup- 
ported in part by a Public Health Services Post Doctoral Fel- 
lowship award, number 1 F32 NS05464. 
Literature Cited 
Becht, G., G. Hoyle and P. N. R. Usherwood 
I960. Neuromuscular transmission in the coxal muscles of the cockroach. 
J. Insect Physiol., 4: 191-210. 
Bowdan, E. 
1977. Walking and rowing in the water strider, Gerris remigis Say. II. Muscle 
activity associated with rapid and slow mesothoraeic leg movement. 
J. comp. Physiol., in press. 
