106 INVERTEBRATE PHYSIOLOGY 



It is probably at this point in the story that the click mechanism, so beau- 

 tifully demonstrated by Boettiger in Diptera, begins to be important. 

 Weis-Fogh (unpublished) has recently obtained some results which help 

 greatly for an understanding of the origin of this peculiar feature of the 

 wing articulation. He has shown that in the thorax of Schistocerca the re- 

 lationship between applied force and wing displacement is not linear in 

 either wing, and that in the hind wing there is a range of instability which 

 amounts to a click mechanism ; the fore wing shows what may well be an 

 earlier evolutionary stage. This demonstration of a click mechanism in an 

 insect whose flight muscles are of the 1 :1 type suggests that even here we 

 may have some measure of deactivation by release in the flight muscles. 

 The instability has the obvious function of increasing the velocity of wing 

 movement above that obtainable with a simple lever action, but it may also 

 assist muscular relaxation. If it could be confirmed that there is a similar 

 instability in the lepidopteran wing articulation, the difficulty of a partial 

 tetanus in the flight muscles would be resolved ; for the deactivation pro- 

 cess could remove the tetanic tension during the phase of elongation of the 

 muscle, and the redeveloped tension once deactivation had worn off would 

 assist the true twitch tension in the next stroke. 



The evolutionary picture is thus of an increase in the neurogenic fre- 

 quency of wing beat beyond the limits set by tetanic fusion in the muscles, 

 with partial deactivation by release ensuring separate contractions. From 

 this to a myogenic rhythm is a small step. Once the myogenic alternation is 

 assured, the motor nerve impulse frequency can drop back to a low level, 

 with a continued evolution of high-frequency muscular activity. The articu- 

 lation mechanics now continue to evolve in some orders to allow increas- 

 ingly isometric contraction of the flight muscles, apparently a necessity for 

 the very high-frequency wing beats of the smaller Diptera and Hymenop- 

 tera. In the higher Diptera the indirect flight muscles do not shorten when 

 detached from the exoskeleton (Tiegs, 1955), and give the appearance of 

 being inexcitable by electrical stimuli applied in the mutilated thorax whose 

 elastic properties have been disturbed. The Homoptera have not evolved 

 so far in the direction of isometry ; Tiegs (1955) finds that in the jassid 

 Enrymela faradic stimulation produces an easily visible shortening of the 

 tergosternal flight muscles, although they have the typical fibrillar struc- 

 ture of myogenically rhythmic muscles. 



Parenthetically it is interesting to consider the cicadas, whose flight 

 muscles, unlike most of the Homoptera, are of the 1 :1 type. Tiegs (1955) 

 emphasizes that in Cyclochila (Cicadidae), as in Siphanta (Flatidae), the 

 flight muscles are intermediate in histological structure between normal 

 insect muscles and the fibrillar type. Cicadas and other Homoptera are 

 known (Snodgrass, 1927) to have a peculiar anatomical arrangement of 



