THE EMBRYOLOGY OF THE BLACK WIDOW SPIDER, LATRODECTUS MACTANS (FABR.)

Abstract

The embryonic development of Latrodectus mactans, the black widow spider, from the time the egg is laid until the embryo is ready to emerge is described and an attempt made to correlate the internal developmental changes with changes in external body form. In agreement with observations of early students of spider embryology it is found that vitellocytes give rise to the mesenteron and hence the yolk cells are considered as the entoderm. The first blood cells are derived from extraembryonic cells, and are therefore ectodermal. A theory of head segmentation is suggested that contrasts with commonly accepted theories of arthropod segmentation by postulating that the spider brain is a syncerebrum made up of a protocerebrum, a greatly reduced tritocerebrum (= stomodaeal bridge), and cheliceral ganglia. The events in the development of the various systems in general closely parallel those described for many other species of spiders, but differ in some important respects from those described for another species of Latrodectus, namely Latrodectus hasselti Thorell.