In his recent review of the developing theory of insect-plant interactions, Gilbert (52) identified four major thrusts in research: insect-plant coevolution, host plants as islands, plant apparency and chemical defense, and resource predictability versus evolutionary strategies of insects. From his review it is evident that, with a few exceptions (23, 45, 53, 64), devdoping theory is addressing primarily a two trophie levd system. In reality, of course, all terrestrial communities based on living plants are composed of at least three interacting trophic levels: plants, herbivores, and natural enemies of herbivores. We argue that theory on insect-plant interactions cannot progress realistically without consideration of the third trophic level. A closer look at the mechanisms of interactions reveals a paradox, and plants have many effects, direct and indirect, positive and negative, not only on herbivores but also on the enemies of herbivores. The third trophic level must be considered as part of a plant's battery of defenses against herbivores.

Speciation in animals is almost always envisioned as the split of an existing lineage into an ancestral and a derived species. An alternative speciation route is homoploid hybrid speciation in which two ancestral taxa give rise to a third, derived, species by hybridization without a change in chromosome number. Although theoretically possible it has been regarded as rare and hence of little importance in animals. On the basis of molecular and chromosomal evidence, hybridization is the best explanation for the origin of a handful of extant diploid bisexual animal taxa. Here we report the first case in which hybridization between two host-specific animals (tephritid fruitflies) is clearly associated with the shift to a new resource. Such a hybrid host shift presents an ecologically robust scenario for animal hybrid speciation because it offers a potential mechanism for reproductive isolation through differential adaptation to a new ecological niche. The necessary conditions for this mechanism of speciation are common in parasitic animals, which represent much of animal diversity. The frequency of homoploid hybrid speciation in animals may therefore be higher than previously assumed.

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Discriminant function and cluster analyses were performed on 19 morphometric variables of the aculeus, wing and mesonotum to determine whether populations of Anastrepha fraterculus (Wiedemann) from different parts of Mexico could be distinguished from populations from South America. Samples were collected from seven localities across Mexico, two from Brazil, and one each from Colombia and Argentina. Results showed there were statistically significant differences between Mexican and South American populations with respect to the aculeus (tip length, length of serrated section, mean number of teeth) and wing (width of S-band and connection between S- and V-bands). The degree of morphological variation observed among Mexican populations was extremely low, and as a consequence, the Mexican populations were identified as a single morphotype by discriminant analysis. The 'Andean morphotype'consisting of the Colombian population, and the 'Brazilian morphotype'consisting of the two Brazilian populations plus the single Argentinian population were also distinguished. It was concluded that the macro-geographical morphotypes from Mexico, Colombia, and Brazil plus Argentina correspond to three distinct taxonomic entities. Comparisons of results with those obtained from behavioural, karyotypic, isozyme and DNA studies suggest that sufficient evidence now exists to name a new Mexican species from within the A. fraterculus complex. This will be done in a separate publication. A provisional key to the morphotypes of A. fraterculus studied is provided.

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Anastrepha obliqua (Macquart) (Diptera: Tephritidae), the West Indian fruit fly, is a frugivorous pest that occasionally finds its way to commercial growing areas outside its native distribution. It inhabits areas in Mexico, Central and South America, and the Caribbean with occasional infestations having occurred in the southern tier states (California, Florida, and Texas) of the United States. This fly is associated with many plant species and is a major pest of mango and plum. We examine the genetic diversity of the West Indian fruit fly based on mitochondrial COI and ND6 DNA sequences. Our analysis of 349 individuals from 54 geographic collections from Mexico, Central America, the Caribbean, and South America detected 61 haplotypes that are structured into three phylogenetic clades. The distribution of these clades among populations is associated with geography. Six populations are identified in this analysis: Mesoamerica, Central America, Caribbean, western Mexico, Andean South America, and eastern Brazil. In addition, substantial differences exist among these genetic types that warrants further taxonomic review.

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