1963] Eisner , Hurst and Meinwald — Defense Mechanism 
95 
beetle is grasped gently by its front end, it can be induced to discharge 
one or more times simply by tightening the grasp intermittently or 
by pinching individual legs with forceps. When viewing such a beetle 
ventral side up with a stereomicroscope, it becomes evident that the 
secretion does not emerge as a liquid ooze, but is expelled from, each 
opening as a jet of finely dispersed spray. At the moment of discharge 
there is seen to project from each glandular pore a short slender nozzle 
(Plate 9, fig. i), from the tip of which the spray shoots forth. By 
Text fig. 1. Diagrams of Chlaenius, showing how the beetle aims its spray 
by bending the tip of the abdomen. At a, the beetle is at rest; at b, the dis- 
charge is in response to stimulation of a hind leg; at c, the target is a stimu- 
lated middle or anterior leg. 
prodding or pinching first one leg and then another, it becomes clear 
that the spray is not ejected in a fixed direction, but is aimed with 
some accuracy toward the particular appendage stimulated. Aiming 
is determined by the degree of flexion of the abdominal tip. When 
anterior legs are stimulated, the tip bends downward sharply, so that 
the projecting nozzles point forward almost horizontally. When 
middle or hind legs are stimulated, the bending is less pronounced, 
and the nozzles point downward at an angle (Text fig. i). Also 
apparent was the fact that the discharge is not necessarily from both 
glands at once. When the stimulus is a unilateral one (e.g. the pinch- 
ing of a leg) only one nozzle is seen to evaginate and spray, and this is 
invariably the one corresponding to the side of the body stimulated. 
Additional experiments were designed to determine more precisely 
the accuracy of aiming. The technique employed was the same as 
used previously with other arthropods that spray (Eisner, 1958a, 
1958b, 1960a; Eisner et al., 1959, 1961). Individual beetles were 
attached to rods and placed on sheets of indicator paper impregnated 
with a chemical mixture that discolors in the presence of the secretion, 
thus enabling a visualization of the spray through the pattern of spots 
