Edwin F. Cook

The Role of Drift Dispersal in Production Ecology of a Stream Mayfly

The role of drift behavior in the production ecology of the mayfly Tricorythodes atratus McDunnough was investigated in a headwater reach of the Mississippi River in northern Minnesota, USA. Comparison of drift and bottom samples revealed that larger nymphs exhibited a greater pro- pensity to drift; in bottom samples, small immatures (1-2 mm) were predominant in a study riffle, whereas larger nymphs (>2 mm) were relatively more abundant in a downstream pool. Annual production was 8.56 gm 2. yr~-2 -I (dry mass) and the annual turnover ratio (annual production/annual mean biomass) was 26; the species was bivoltine. Errors in production rate estimation resulting from sampling a single habitat were substantial when differential size dispersal of nymphs occurred.

Effects of Pregnancy on the Spleen in Mice

Spleen size was analyzed in 2368 wild white-footed mice, Peromyscus leucopus , in Minnesota. Sex, season and type of death (chloroform or killer trap) did not affect spleen size. Females in late pregnancy showed an average increase of 50% in size of spleen. The cells in the nodules of spleens of pregnant mice were found to have large vesicular nuclei. It is suggested that this may be a reaction to an antigen produced by the embryos. Normal spleen size varied greatly (15–72 mg in animals of the same size), the S.D. being 13.6. Spleen weight versus body length is a straight line.

Live cycles, drift, and standing stocks of some stoneflies (Insecta: Plecoptera) from streams in Minnesota, U.S.A.

From analyses of monthly length frequency distributions of stonefly (Plecoptera) nymphs from stream riffles, two types of univoltine life cycles (Hynes 1961, classification) were observed: slow seasonal forIsoperla transmarina, Isoperla signata, Isogenoides olivaceus, andAllocapnia rickerii, and fast seasonal forProstoia similis. The Perlodidae stoneflies (Isoperla andIsogenoides) showed greatest propensities to drift near the end of their life cycles. Drift rates of the other two species of stoneflies were correlated to standing stock estimates.

Life cycles, standing stocks, and drift of some Megaloptera, Ephemeroptera, and Diptera from streams in Minnesota, U.S.A.

From analyses of monthly length frequency distributions, univoltine life cycles were described for Sialis dreisbachi, Stenonema vicarium, and Dicranota sp. from stream riffles. Stream drift of immature S. dreisbachi and S. vicarium peaked near the ends of their life cycles in the spring. Drift of these two species was not related to standing stocks. Dicranota larvae exhibited negligible drift rates.

The immature stages of the genus Hoplopleura (Anoplura: Hoplopleuridae) in North America, with descriptions of two new species

The immature stages of the genus Hoplopleura are but little known. Of the known North American species only a few have been described or illustrated. Ferris (1921, 1951) illustrated 1 stage of H. acanthopus (Burm.) and H. arboricola K. and F., and Pratt and Karp (1953) illustrated all stages of H. oenomydis Ferris. In addition to this, a brief description of 1 stage of H. hirsuta Ferris (Ferris, 1921) and H. oryzomydis P. and L. was made by Pratt and Lane (1951). In the course of collecting specimens for quantitative studies on louse populations, numerous immature stages have come to hand representing most of the described North American species. All stages of North American species have been collected by us with the exception of H. oenomydis, H. hirsuta and H. oryzomydis. Specimens representing H. oenomydis have been made available to us through the courtesy of Dr. Harry Pratt of the C.D.C. in Atlanta. Skins of Oryzomys palustris and Sigmodon hispidus from which H. hirsuta and H. oryzomydis were recovered were made available to us by Dr. Norman Negus of Tulane University. The adult stages have all been carefully described and precisely illustrated by Ferris (1921, 1951), and also to some extent by Pratt and Lane (1951). Keys to the adult stages are also available in Ferris (1951) and Pratt and Lane (1951). In the present paper, in addition to the key and descriptions of all immature stages, 2 new species are described, 1 of which is readily distinguished in both adult and nymphal stages and another which can be distinguished only in the immature stages. Characters are also considered which will more readily permit the separation of H. arboricola from H. erratica in adult stages. The several nymphal instars are easily recognized, since in any long series of specimens at least a few nymphs will be found on the point of molting. Succeeding instars are at this time visible within the nymphal skin of the preceding stage. It is apparent in this material that there are 3 nymphal stages. It is evident that the North American species of the genus are separable into 3 groups. Those nymphs found on chipmunks and tree squirrels (H. arboricola, H. erratica, and H. sciuricola) have 5 visible pairs of minute abdominal spiracles. Those found on cricetid rodents have no such spiracles visible in any immature stages. The 1 species found on the flying squirrel (H. trispinosa) lacks abdominal spiracles as a nymph, but has the anal lobes apically setose in all stages, a condition not evident in the other 2 groups. The lice of chipmunks and tree squirrels also have conspicuous dorsal and ventral abdominal setae in addition to the marginal