CONTROL OF MOLTING IN MANDIBULATE AND CHELICERATE ARTHROPODS BY ECDYSONES

¹ Department of Biology, Marquette University, Milwaukee, Wisconsin 53233 and Developmental Biology Laboratory, School of Biological Sciences, University of California, Irvine, California 92664 and Marine Biological Laboratory, Woods Hole, Massachusetts 02543

1. The ability of ecdysones to induce molting in arthropods other than insects was examined in representatives of both the mandibulate and chelicerate subphyla.

2. All five ecdysones tested caused molting in the fresh-water crayfish, Procambarus. Their relative activities were: ecdysterone > inokosterone > ecdysone > ponasterone A > cyasterone. Doses as low as 3 µg/g of ecdysterone caused 100% of all test crayfish to undergo apolysis and secrete a new cuticle within 14 days after injection, but only in a few cases did the animals shed their old cuticles spontaneously after experimental treatment. At higher doses the new cuticle was thinner than normal and had abnormal bristles. The stimulation of molting was specific for ecdysones and was not copied by a variety of ecdysone analogues or other steroids.

3. Histological and autoradiographic studies revealed that ecdysterone at the levels used in these experiments caused molting in crayfish without DNA replication.

4. Ecdysterone also caused molting in the marine fiddler crab, Uca pugilator.

5. Ecdysterone caused molting in several chelicerate arthropods including the spider, Araneus cornutus, the tarantula, Dugesiella hentzii and in the horseshoe crab, Limulus polyphemus, which is among the most primitive of all living arthropods.

6. In spiders the ecdysone caused behavioral effects before any obvious changes in the cuticle were evident.

7. In almost all cases, molts induced by ecdysone were characterized by abnormal cuticles similar to those produced by injections of ecdysones in insects, a result which suggests a direct action of ecdysone on the epidermal cells. Most of the experimental animals failed to survive the molt and few underwent spontaneous ecdysis. These effects probably result from the abnormal delivery of a large amount of hormones in one dose, in contrast to the gradual release of hormone in situ.

8. From an analysis of all of the dose-effect data, it is concluded that almost all of the ecdysones which are active in one group of arthropods will have some activity in other groups. Since spiders and horseshoe crabs require doses about ten times as high as those needed for mandibulate arthropods, the specific ecdysones employed by the chelicerates may differ from their counterparts in insects and crustaceans.

9. The evidence suggests that ecdysones are the normal molting hormones of all arthropods and supports the view that arthropods have a common ancestry and are not a polyphyletic group.

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