Misee/huicous. 337 



such an apparatus, the life-history of these crabs must first be briefly 

 noticed. 



They are gregarious, and though at times they may bo seen 

 marching (migrating ?) in battalions across the sand, they usually 

 live in " warrens " at and about high-water mark, where they 

 excavate tortuous burrows, methodically turning over the surface of 

 the surrounding sand for any particles of food that it may contain, 

 and when alarmed flying each one to its burrow. Their chief 

 enemy appears to be the common red-and-white kite. Now the use 

 of the stridulating-organ appears to be this, that when a crab has 

 entered its burrow it may, by the utterance of warning notes, 

 prevent other crabs from crowdiug in on top of it. It is easy to 

 imagine that, in the consternation of a sudden hostile surprise, 

 several crabs might fly for refuge to the same burrow, Avith the 

 result that both the lawful occupant and the intruders might be 

 stifled, or crushed, or in some waj' injured, and it is easy to under- 

 stand that the power of uttering a warning signal would protect the 

 lawful occupant from such harm. That, when intrusion does take 

 place, the sound is feared by the intruder, I shall presently show. 



The possible reciprocal advantage to the other crabs from such 

 warning we must pass by, in discussing the origin of the stridulating- 

 organ ; for, although the crabs are gregarious, there appears to be 

 no social co-operation whatever, and we can assume that individual 

 structural modifications exist for the good of all the members of a 

 flock only when there is social co-operation. 



The sounds can be heard, and their eft'ects seen, by forcing one 

 crab, which we will call the intruder, into the burrow of another, 

 which we will call the rightful owner. The intruder shows the 

 strongest reluctance to enter, and will take all the risks of open 

 flight rather than do so, and, when forced in, he keeps as near 

 the mouth of the burrow as possible. When the rightful owner 

 discovers the intruder he utters a few broken tones of remonstrance, 

 on hearing which the intruder, if permitted, will at once leave the 

 burrow. If the intruder be prevented from making his escape, the 

 low and broken tones of the rightful owner gradually rise in loud- 

 ness and shrillness and frequency until they become a continuous 

 low-pitched whirr, or high-pitched growl, the burrow acting as a 

 resonator. 



One has often to wait quietly for a long time, until the alarm of 

 the a])pearance of an enemy has subsided, before the rightful owner 

 discovers the intruder and begins to stridulate ; and had it not been 

 for a happy accident I should not this season have repeated experi- 

 ments that three years ago, owing to my impatience, were 

 unsuccessful. I need hardly say that what little credit there is in 

 this discover}' is entirely due to Professor Wood-Mason, who 

 directed my attention to the subject, and who has stores of knowledge 

 accumulated upon stridulation in crustaceans. And in the ' Descent 

 of Man,' p. 274 (l!nd edition), there is a reference to Hilgendorf's 

 discovery of jtossible stridulating-organs in the higher Crustacea. — 

 From the Administration lieport of the Marine Survi'i/ of India for 

 18<Jl-92. 



Ann. ih Mafj. N. Hist. Scr. 6. Vo/. x. 23 



