THORAX, LEGS, AND WINGS. 
37 
scribed by many observers, and latterly by Dahl (28), 
Simmermacher (156), and Rombouts (143), the first 
two observers more particularly turning their attention 
to the method of attachment of the feet to smooth 
surfaces. Dahl says the six legs are necessary as 
climbing organs ; one leg will always be perpendicular 
to the plane when the animal is moving up a vertical 
surface. As we know that three is the smallest 
number with which stable equilibrium is possible, an 
insect must have twice that number. 
For moving on rough surfaces bees use their 
claws (Fig. 17, m ni)., which by their shar]^ tips are 
able to enter the smallest depression Sy 
and so obtain a firm hold ; but as these 
would slip on smooth surfaces, such as 
glass, another organ of fixation is pro- 
vided. This organ, is situated be- 
tween the claws, and is called the 
pulvilliis. 
It is a fleshy lobule and almost 
smooth on the under surface, which 
gives out an adhesive secretion and 
causes it to adhere to a smooth sur- 
face. The upper surface is covered 
with very minute hairs, and when not 
in use the pulvillus is folded up and 
the claws. Within there is an elastic rod (seen 
shaded in the illustration, which is taken from a 
photo-micrograph of the real foot), so that when this 
is brought down the pressure expands the pulvillus 
and spreads it over the smooth surface, the secretion 
Fig. 17. 
Pulvillus and 
Claws. 
stands above 
