At Akayu-machi and Takahata-machi (the north-eastern part of Yonezawa Basin), Higashiokitama-gun, Yama-gata-ken, Japan, we can see thin white threads and small white masses flow quietly in the blue sky on a fine and windless (or breezy) day, from the end of October to December. It will snow soon after this phenomenon, so people in that district call it “ushering in snow” (Yukimukae).
This phenomenon is the same as gossamer (England) or Babie late (Poland), and is found to by caused by threads of spiders as they migrate in the air. It is only in this district in Japan that spiders are observed to migrate in the air and are given a special designation.
I have observed “ushering in snow” and collected spiders for the past fifteen years.
According to my observation, spiders climb to the tip of the leaves of grass or the tip of sticks toward the end of October (occasionally during September), stretch their legs, raise their hips, and emit the threads. Before the threads are emitted, active movement of spinneret was noticed; that is, spiders shook violently the posterior spinneret and repeated the actions which seemed to serve to spin the threads out from the middle spinneret. The threads are emitted from the middle spinneret first. The anterior and posterior spinneret open outside. It was also observed that the number of the threads increased rapidly before spiders flew up, amounting to ten odd. (On such an occasion, the strings are emitted from the both the anterior and the posterior spinneret, centering around the middle spinneret.) On no occasion did they use their legs.
I recognized twelve families, thirty-nine species of “ushering in snow”, that is, gossamer spiders, and it is easily predicted that the kinds will be increased by further investigation.
Among these spiders, it was found that those that are migratory far outnumber those that are web-spinning, and those spiders that migrate far are mostly migratory.
I have classified these spiders in terms of “their ways of life” and “the stage in their life for flight (young, sub-adult, adult)”, shown them in the graph, and investigated the evolution of spiders.
Spiders migration and scattering with the thread flowing in the air seem to bear a close relation to preservation of spiders both as individuals and as a family, and is regarded as an ecological phenomenon against an internecine struggle or lack of food.
Migratory spiders, such as Thomisidae and Lycosidae, emit the threads actively and migrate in the air, in their prime of life. They are neither saltatorial nor web-spinning.
Salticidae have acquired saltation, which has made it easy for them to catch food necessary for preservation of both individuals and a family. Hence, only a few species of this family migrate in the air when they are young.
On the other hand, those spiders which developed ability in spinning a web became capable of getting food without migration, so they did not need to migrate at all. Only during the periods of young and sub-adult when they cannot make big webs do they seem to show the habit of dispersion through migration in the air.
Small-sized spiders, such as Micryphantidae have not been so evolved as to make big webs, they continue a migratory life and presumably keep the habit of migration in the air even as adults.
I have considered the question why spiders migrate toward the end of autumn in relation to lack of food, and reached the following conclusion. The time when they have plenty of food is the time also for their reproduction and egg-laying, and at such a time no spider migrates, while toward the end of autumn when they lack in food, they set out in migration. Furthermore, an air current is crucial for the flight of spiders, and when one spider can fly
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