XVIII A PARASITE— THE COCOON 251 



booty. Then come marches and counter-marches 

 of cunning versus prudence. 



The Bembex drops straight down with an im- 

 perceptible flight, as if she let herself sink gently, 

 making a parachute of her wings. Now she is 

 hovering just above the ground ; the flies take wing, 

 placing themselves one and all behind her, — some 

 nearer, some farther, — in a geometrical line. If she 

 turns round to disconcert them, they turn too, with 

 a precision which keeps them all in the same 

 straight line ; if she advances, so do they ; if she 

 draws back, they draw back too, measuring their 

 flight, now slow, now stationary, on that of the 

 Bembex at the head of the file. They do not 

 attempt to fling themselves on the desired object, 

 their tactics being merely to hold themselves in 

 readiness in the position of rearguard, so as to avoid 

 any hesitation when the rapid final manoeuvre shall 

 come. 



Sometimes, wearied out by their obstinate pursuit, 

 the Bembex alights, and the flies instantly settle on 

 the sand, still behind her, and keep quite still. She 

 rises again, with a sharper hum — the sign no doubt 

 of increasing indignation ; the flies follow her. One 

 last means remains to throw the tenacious Diptera 

 off the track ; the Bembex flies far away — perhaps 

 hoping to mislead the parasites by rapid evolutions 

 over the fields. But the crafty flies are not taken 

 in ; they let her go, and settle down again on the 

 sand round the burrow. When the Bembex returns 

 the same manoeuvres begin again until the obstinacy 

 of the parasites has exhausted her prudence. At a 

 moment when her vigilance fails, the flies are 



