CO-ORDINATED RHYTHM IN INSECTS. 175 



2nd Ed., 1920, p. 93), "All in any locality chirp in unison. 

 Early in the evening, when the chirping first begins, there may 

 be lack of unanimity in keeping time, but this only lasts for a 

 short time — soon all chirp in unison and the monotonous beat 

 of their call is kept on uninterrupted throughout the night. 

 Individual singers will stop to rest, but when they start again 

 they keep time with those that have continued the chorus." 



To the accounts above I can now add the following from my 

 own experience. On several occasions during the past few years 

 I have observed a co-ordinated action in a dark-coloured aphis 

 which is common at certain times of the year on the young 

 leaves of cocoa in Trinidad and other parts of Tropical America. 

 The aphis was identified for me by Mr. Theobald as Toxoptcra 

 cqffeae, Nietner, and is apparently distributed throughout the 

 world wherever coffee or cocoa is found. 



The aphis sits on the undersides of the leaves of cocoa, 

 almost anywhere on the surface but is usually particularly 

 common along the main veins, and when abundant there may be 

 a hundred or more winged and wingless insects in various stages 

 of growth side by side on the leaf. 



When such a colony is disturbed by turning over the leaf to 

 the light or by scraping slightly with the finger nail, a greater 

 or smaller number of the aphids on the leaf set up a regular and 

 rhythmic movement which consists essentially of raising the 

 hind part of the body into the air, keeping the head fixed and 

 the beak still in the leaf tissue. 



The raising and lowering movement occupies abcut one 

 second and it is repeated at intervals of three or four seconds 

 with great regularity, sometimes for twenty or thirty times. If 

 the colony is responsive every apbis on the leaf will be moving 

 in absolute co-ordination and with absolute regularity. 



Sometimes two or three separate movements will be set up 

 on one leaf so that all in one section are keeping time with each 

 other, but not with another section, but more usually all on one 

 leaf move together. Individuals quite isolated would keep 

 regular time with the rest. 



While observing this closely I found that each movement was 

 accompanied by a distinct scraping sound, audible, with a big 

 colony, as much as eighteen inches away from the leaf and clear 

 enough for me to be able to tell when the aphids were moving 

 without seeing them. 



I do not know whether the sound is a deliberate production 

 on the part of the aphis, or whether it is an accidental result of 

 the movement magnified by the leaf surface. I have carefully 

 examined mounted specimens of the aphis with a microscope, 

 but have been unable to find any trace of a stridulating organ. 



That the movement is significant and not accidental is shown 

 by the fact that I have observed it on at least a dozen occasions 



