another, velvet black, with large square cephalothorax and small pointed
abdomen; the third and most abundant kind were of different shades of
olive green, and varied greatly in size, the largest being fully a
quarter of an inch in length. Apparently these spiders had been driven
up from the low ground along the stream where it was wet, and had
congregated along the borders of the dry ground in readiness to migrate.

"25th. Went again to visit the spiders, scarcely expecting to find them,
as, since first seeing them, we have had much wind and rain. To my
surprise I found them in greatly increased numbers: on the tops of
cardoons, posts, and other elevated situations they were literally lying
together in heaps. Most of them were large and of the olive-coloured
species; their size had probably prevented them from getting away
earlier, but they were now floating off in great numbers, the weather
being calm and tolerably dry. To-day I noticed a new species with a grey
body, elegantly striped with black, and pink legs--a very pretty spider.

"26th. Went again to-day and found that the whole vast army of
gossamers, with the exception of a few stragglers sitting on posts and
dry stalks, had vanished. They had taken advantage of the short spell of
fine weather we are now having, after an unusually wet and boisterous
autumn, to make their escape."

Here it seemed to me that a conjunction of circumstances--first, the
unfavourable season preventing migration at the proper time, and
secondly, the strip of valley out of which the spiders had been driven
to the higher ground till they were massed together--only served to make
visible and evident that a vast annual migration takes place which we
have only to look closely for to discover.