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The complete larval development, consisting of two zoeal stages and a megalopa, and the first crab stage is described for the shallow-water western Atlantic spider crab Microphrys bicornulus. Data from laboratory cultures indicate that the species can complete its planktonic development in less than a week, and is able to attain first crab stage in as few as 10 days. The zoeal stages of M. bicornulus show a great many similarities to known zoeae in other genera within the subfamily Mithracinae, including species of the American genus Milhrax, and to Macrocoeloma, and to a lesser extent the Indo-West Pacific genera Tiarinia and Micippa. Morphological features shared among both the zoeal and megalopal stages of the various mithracine genera are compared, and phylogenetic relationships within Microphrys, Milhrax, and Macrocoeloma are proposed.Microphrys bicornulus is a small (ca. 3CM0 mm carapace width, cw) intertidal and shallow subtidal spider crab in the subfamily Mithracinae, which occurs from Bermuda and North Carolina, southward to Brazil (Williams, 1965; Markham and McDermott, 1981). The species is ovigerous throughout the year over a large part of its range, but in spite of its abundance and the ease of its collection there has been no complete study published on the larval development of this crab. Lebour (1944) gave an abbreviated description and illustration of 2 zoeal stages from Bermuda, and Hartnoll (1964) provided an expanded description and illustration of the prezoea and 2 zoeal stages from Jamaica. Neither author's study is sufficiently detailed to allow meaningful comparison with larval stages of other mithracine crabs. Ironically, the complete larval development had been worked out for Microphrys bicornulus by Yang (1967), but the study forms part of a voluminous dissertation which is not readily available, and has remained unpublished. Inasmuch as several discrepancies in description and illustrations occur between the publications of Lebour and Hartnoll and that of Yang, we recultured the larvae of M. bicornulus and compared our material with the detailed study provided by Yang. In this paper we redescribe and illustrate the zoeal, megalopal, and first crab stage of M. bicornulus. We compare the larvae of this species with those known from other genera and species in the Mithracinae, pointing out possible relationships among them.