The shapes, sizes, and chorionic ornamentation of mature oocytes/eggs are described along with ovariole and mature oocyte numbers of six lineages of primarily South American cleptoparasitic bees. This information is related to whether the eggs are introduced into brood chambers that are still open and being provisioned by the host female or whether the chambers have already been closed by the host females. The lineages, all in the Apidae, are as follows: (1) Kelita (Nomadinae: Brachynomadini), (2) Isepeolus and Melectoides (Apinae: Isepeolini), (3) Leiopodus (Apinae: Protepeolini), (4) Rhathymus (Apinae: Rhathymini), (5) Mesoplia and Epiclopus (Apinae: Ericrocidini), and (6) Exaerete (Apinae: Euglossini).

A table in the section on Discussion and Analyses summarizes information on mature oocyte/egg size (egg index), total number of mature oocytes, mature oocytes per ovariole, and ovariole number (ovariole formula) for all taxa of cleptoparasites, worldwide, that have been studied to date. It shows that almost all of the Nomadinae have more than the plesiomorphic number of ovarioles, a feature also found in two of the three studied genera of the Ericrocidini. All other cleptoparasitic lineages lack extra ovarioles. The potential selective advantage of extra ovarioles is discussed. Also discussed is whether the large number of mature oocytes carried by cleptoparasites might result, in part, from the length of time required for chorion deposition after the oocytes reach maturity.

The table shows not only that the mature oocytes/eggs of cleptoparasitic bees in general tend to be smaller than those of solitary bees, but that the mature oocytes/eggs of those cleptoparasites that hide their eggs in open host brood cells are significantly smaller than those that introduce their eggs into cells that have been closed by the host. The potential selective advantages of small egg size in cleptoparasitism are explored.

Lastly, the unusual modified shapes of mature oocytes/eggs and the thick chorions of cleptoparasites that oviposit in open host cells are attributed to ways of protecting the eggs from discovery and damage by returning host females.

Appended is a scanning electron micrograph of the micropyle of the North American Stelis elongativentris Parker (Megachilidae: Anthidiini), the ovariole and oocyte statistics of which have been published earlier. Also appended are a description and illustrations of the oocyte of Coelioxys novomexicana Cockerell (Megachilidae: Megachilini).