The role of temperature and larval crowding in morph determination in a tropical beetle, Callosobruchus subinnotatus.

Many insect species can produce individuals of more than one form or morph. Different morphs of the same species may differ in their physiology, morphology and in behaviour. Understanding the factors and mechanisms involved in determining the production of different morphs of insect species is of major importance in understanding the evolution of specific life-history strategies. In this paper, we studied the importance of temperature as an environmental factor involved in morph determination of the tropical beetle, Callosobruchus subinnotatus. Adults occur as relatively sedentary, highly fecund, 'normal' morphs or as an 'active' morph adapted to dispersal. Larval crowding, seed density and external temperature were independently manipulated in a series of controlled experiments and the proportions of 'active' and 'normal' adult morphs among the emergent adult populations were quantified and compared. Development in crowded conditions was found to be associated with the production of a significantly higher proportion of 'active' adults than development in isolation, and was also responsible for a predictable rise in the localised temperature of infested heaps of seeds of between 4 and 8 degrees C above ambient (27 degrees C). This rise in temperature is subsequently shown to be directly and quantitatively associated with the proportion of 'active' adults among emerging adults, both in the presence of larval crowding and independently from it. Thus, it is suggested that in the crowded environment representative of an infested seed store, it is rising temperature, occurring at a specific point in insect development which is the proximate cue for 'active' morph induction in C. subinnotatus. The results are compared to the strategies used by other polymorphic insects to survive in heterogeneous environments.

Variation in responses to susceptible and resistant cowpeas among West African populations of Callosobruchus maculatus (Coleoptera: Bruchidae).

The cowpea seed beetle, sometimes also known as the cowpea weevil, Callosobruchus maculatus (F.), is a major pest of stored cowpeas (Vigna unguiculata Walpers) in West Africa. Control methods have included development of 'resistant' varieties as an environmentally benign alternative to insecticides, but there is concern over their effectiveness because of population variation among the insects and the possibility of adaptation overcoming seed resistance. Populations of C. maculatus from Nigeria, Ghana, Benin, and Niger, were used to examine variation in response to resistant and susceptible cowpea varieties at two geographical scales. Among seven Nigerian populations, there were significant differences in development times, the pattern of adult emergence, adult weights, and female fecundity when reared under identical conditions. Development in the resistant variety was retarded, produced higher mortality and lower adult weights. Significant interactions between variety and population were evident in terms of their effects on adult weight and development time; development times in the resistant variety were longer and emergences occurred over a longer period in some populations than in others. Population responses to resistant seeds were therefore unpredictable, but there was no evidence to suggest adaptation to overcome seed resistance within three generations. On a larger geographical scale, variation in performance was much greater and therefore, even less predictable. Mortality in resistant seeds was also higher among populations collected from outside Nigeria and may be explained by significant adaptation among Nigerian populations to previous release of resistant varieties. The findings are discussed in relation to understanding the extent of intraspecific variation in C. maculatus and its implications for future pest management.

Bionomics and polymorphism in Callosobruchus subinnotatus (Coleoptera: Bruchidae).

Callosobruchus subinnotatus (Pic) is the major insect pest of stored bambara groundnuts, Vigna subterranea (L.) Verdcourt, in sub-Saharan West Africa, but little is currently known about its biology or how it may be controlled. A series of laboratory studies was performed to investigate the bionomics of and differences between two apparently different morphs of adult of each sex of this species, here termed 'active' and 'normal'. Major differences in their morphology, physiology and behaviour were identified and are described in detail for the first time. They provide clear evidence of the existence of an adult polymorphism among populations of this species, which is comparable in certain respects to that previously described for C. maculatus (Fabricius) and C. chinensis Linnaeus. Adults can be separated into the correct morph based on characteristic differences in elytral and pygidial colour and pattern. 'Normal' adults are characterized by having high fecundity, short adult life and are relatively sedentary while 'active' adults exhibit reproductive diapause (suspension of reproductive activity), are long lived, and show (at least in females) increased dispersal tendencies. These characteristics suggest adaptation of the 'active' and 'normal' morphs respectively to the different environments of field and seed stores, and the significance of the polymorphism in the life history of C. subinnotatus is discussed in this context. The design of any effective control regime for this bruchid needs to take account of and could potentially be based upon the existence of polymorphism in C. subinnotatus.

Evidence of allergic hypersensitivity to chironomid midges in an English village community.

Some chironomid (non-biting) midges contain potent human allergens, principally in the form of insect haemoglobins. In view of previously demonstrated immunological cross-reactivity between some different midge species, the possible role of British midges in allergic disease was studied. The chosen site, Farmoor near Oxford, is a village that has been subject to several annual nuisance midge swarms since the construction of an adjacent water-supply reservoir in 1964, which was extended in 1975. The incidence and severity of hypersensitivity in the community was assessed by questionnaire survey, skin-prick testing, radio-allergosorbent tests (RAST) and RAST inhibition, and enzyme-linked immunosorbent assays (ELISA) and ELISA inhibition, using extracts of locally caught adult midges (Tanytarsus sylvaticus, T. bathophilus and Chironomus anthracinus) and laboratory cultured larvae (C. riparius). These tests revealed a low but significant level of midge-related hypersensitivity with skin-prick results showing seasonal fluctuations, but this seasonality was not confirmed with an IgE RAST. A correlation between IgE levels and relevant symptoms was found (P less than 0.01) using RAST, but not between IgE and skin-test results. Enzyme-linked immunosorbent assay results did not correlate with any other parameter measured. The incidence and severity of allergic reactions to the midges tested in this community was low.

Organization of the cuticle of an aquatic fly larva.

The aquatic, apneustic larva of the midge, Chironomus riparius, has a very thin (up to 5 micrometers), readily deformable, post-cephalic cuticle. The ultrastructure of this cuticle from newly moulted and older final instar animals, and exuvia shed at pupation, has been examined using routine methods and also after the extraction of proteins with formamide and acetic acid. From the results described, and using established criteria, it is inferred that an exocuticle is present and represents about 25% of the thickness of the mature procuticle, the remainder being endocuticle. Therefore, it would seem that this exceptionally delicate cuticle conforms to the conventional plan of tanned or sclerotized solid cuticles, unlike those reported in the larvae of cyclorrhaphous dipterans such as blowflies or other soft-bodied insects. This is the first account, using experimental techniques, of the fine structure of sectioned cuticle from nematocerous dipteran larvae. It also indicates the value of the exuvium as a source of information about cuticle structure.

The ultrastructure of the non-neurosecretory components in the brain of the midge, Chironomus riparious Mg. (diptera: nematocera).

The ultrastruct of the neural sheath, glial cells and neurons in the brain of the neoimaginal male Chironomus riparius is described. The neural sheath comprises a neural lamella and underlying perineurium. The neural lamella consists of an amorphous matrix in which fine fibrils occur. The perineurium is composed of two cell types forming a continuous layer around the brain. The subjacent cortical layer, composed of the cell bodies of neurons and glial cells, varies considerably in thickness and surrounds the centrally located neuropiles. Three types of glial cells are distinguished on the basis of their positions and appearances. Five types of neurons are described which differ in size and relative frequency of organelles. Four types of axons, including those of neurosecretory cells, are distinguished by their size and content.

An ultrastructural study of the larval integument of the midge, Chironomus riparius Meigen (diptera: chironomidae).

The larval integument of the midge, Chironomus riparius Mg., is unusually thin although it conforms with the normal insect pattern. The cuticle of the post-cephalic segments is about 3 micrometer thick and overlies an epidermis which has an irregular basal plasma membrane resulting in spaces occurring between it and the basement membrane. The ventral tubuli have a similar epidermis but the cuticle is somewhat thinner. The anal papillae have the thinnest cuticular covering with a uniquely folded epicuticle of variable thickness, and their epidermis has the characteristics of a transporting epithelium. No evidence of pore canals could be found in the cuticle of any part except the head capsule which has a remarkably smooth epicuticle and a distinct layer which may represent the exocuticle. There are no spaces between the basement membrane and basal plasma membrane of the epidermis in the head. Ultrastructural evidence would suggest that gaseous exchange can occur across most of the post-cephalic integument.

A structural study of the anal papillae of the midge Chironomus riparius Meigen (Diptera: Chironomidae).

Larvae of the midge, Chironomus riparius Mg., have four anal papillae arranged as two pairs, one dorsal to and the other ventral to the anus. Structural study with light and electron microscopes has revealed that their integument consists of a thin cuticle overlaying a thick, syncytial epidermis which is specialised to facilitate ion transport. There is a distinct neck or collar region at the junction of each papilla with the rest of the body wall. Although in many respects these structures resemble those in mosquitoes, there are both morphological and physiological features which suggest that differences exist in the means by which ion exchange with the environment is controlled and effected in the two families.