Box 71 How do entomologists recognize insect species

Although there are a myriad of ideas on what constitutes a species, most species concepts center on sexually reproducing organisms and can be applied to most insects. Different species concepts adopt different properties, such as phenetic distinctness, diagnosability, or reproductive incompatibility, as their species criterion. The use of various defining properties for species has led to diasagreements over species concepts, but the actual practice of deciding how many species exist among a sample of related or similar-looking specimens generally is less contentious. Systematists mostly use recognizable discontinuities in the distribution of features (character states) and/or in the distribution of individuals in time and space. The recognition of species is a specialist taxonomic activity, whereas identification can be performed by anyone using the appropriate identification tools, such as dicho-tomous keys, illustrated computer interactive keys, a reference database, or, for a few pest insects, a DNA probe. Insect systematists use a variety of data sources - morphology, molecules (e.g. DNA, RNA, cuticular hydrocarbons, etc.), karyotypes, behavior, ecology and distribution - to sort individuals into groups. Traditionally, differences in external appearance (e.g. Fig. 5.7), often coupled with internal genitalic features (e.g. Fig. 5.5), were used almost exclusively for discriminating insect species. In recent decades, such specific-specific variation in morphology is frequently supported by other data, especially nucleotide sequences from mitochondrial or nuclear genes. In insect groups with morphologically similar species, particularly sibling or cryptic species complexes (e.g. the Anopheles gambiae complex discussed in Box 15.3), non-morphological data are important to species recognition and subsequent identification.

DNA barcoding has been promoted by some scientists as a molecular diagnostic tool for species-level identification (see section 17.3.3) and a few have advocated its use for species discovery. The standard barcode is a region of mitochondrial DNA, but nucleotide sequences from other genes can be used similiarly for species delimitation. Two criteria can be used for species recognition using sequence data: (a) reciprocal monophyly, meaning that all members of a group must share a more recent common ancestor with each other than with any member of another group on a phylogenetic tree, and (b) a genetic distance cut-off, such that interspecific distances are greater than intraspecific distances by a factor that is different for each gene (e.g. 10 times the average intraspecific distance has been proposed for species delimitation using COI data). In practice, recognizing species reliably in this way requires thorough taxon and specimen sampling to avoid underestimating intraspecific variation through poor geographic sampling, or overestimating interspecific distance through omission of species. However, even if sampling is inadequate, such molecular data can provide clues to possible species segregates and thus allow the re-interpretation of existing morphological data by highlighting the phylogenetically informative anatomical characters.

The woodroaches (Blattodea: Cryptocercus) of eastern North America provide a good example of an insect group that has been studied taxonomically using a variety of data sources. All populations within the Cryptocercus punctulatus species complex in the Appalachian mountains are morphologically, behaviorally, and ecologically similar, but have been described as four species based on unique male diploid chromosome numbers (2n = 37, 39, 43, or 45) and some diagnostic bases from two rRNA genes. Consistent morphological differences in the reproductive structures, especially of adult females, also support these species. However, there appear to be five taxa (as shown in the tree, after Everaerts et al. 2008) based on evidence from recent analysis of data from three genes supported by cuticular hydrocarbon data (see section 4.3.2 for information on cuticular hydrocarbons). The latter two datasets differ from the karyotype information in indicating two distinct and not closely related clades (or taxa) with 2n = 43, indicating parallel evolution of that karyotype perhaps by convergent reduction of chromosome number. Woodroaches with 2n = 43 (currently C. punctulatus sensu stricto) occur over a wide geographic range compared with the other species, and variation in reproductive structures has not been examined across the range. Not all groups recognized by the hydrocarbon data are fully concordant with those delimited by other evidence, since some woodroaches in two clades that are distinct genetically and either 2n = 43 or 2n = 45 have similar hydrocarbons, plus there are two distinct hydrocarbon groups within 2n = 45 woodroaches. Thus hydrocarbon information fails to support two of the putative Cryptocercus species but fully supports other species. At least in Cryptocercus, it seems that differentiation of cuticular hydrocarbons can occur with minimal karyotype and genetic change, and changes in genes coding for cuticular hydrocarbons do not necessarily accompany changes in other genes or chromosome number.

In termites, which can be difficult to identify morphologically (particularly based on workers), there is some convincing evidence that hydrocarbon profiles can differentiate species. In a few studies that report more than one hydrocarbon phenotype for a named termite species, other sources of evidence (such as intercolony agonistic experiments and genetic data) suggest that these phenotypes are distinct taxa or species. However, data must be interpreted with care because there might be an environmental influence on hydrocarbon composition, as found for Argentine ants fed on different diets. Also a detailed behavioral study of an African Mastotermes species showed that although the level of mortality due to aggressive encounters increased with differences in cuticular hydrocarbons between colonies, termites displayed lower aggression to neighboring colonies than to more distant ones regardless of hydrocarbon phenotype. Clearly more studies are needed of different types of insects using multiple data sources, including behavioral bioassys, to test for concordance between hydrocarbon phenotypes and taxonomic groups.

A novel data source for species delimitation that is unlikely to have an environmental component is the neuropeptide profile of a species. These diverse chemical messengers are produced in the nervous system and exist as neuropeptide families containing multiple forms. Minute quantities of expressed peptides (rather than their coding genes) can be extracted from the major neuro-hemal organs (corpora cardiaca plus thoracic and abdominal perisympathetic organs) of insects and analyzed directly using modern mass spectrometric techniques. The first use of neuropeptides in insect taxonomy explored peptide mass fingerprints for selected members of the Mantophas-matodea, in which nearly half of the 32 peptides analyzed differed among species, with species known to be closely related having similar mass fingerprints, and all specimens of one species collected from different locations having identical peptides. Recent advances in mass spectrometry (due to unrelated research in proteomics) make it possible to screen for neuropeptides relatively quickly, at least in medium- and large-sized insects from which it is possible to dissect the neuro-hemal organs.

Strict consenus tree of the Cryptocercus punctulatus species complex from 22 locations, based on combined datasets of mitochondrial DNA and nuclear DNA. Nodes with strong support are indicated by an asterisk, and the five putative species are numbered 1 -5. Each terminal is labeled with the collection location name, the male diploid chromosome number, and the hydrocarbon group (HcG).

Despite taxonomic problems at the species and genus levels due to immense insect diversity and phylogenetic controversies at all taxonomic levels, we are moving towards a consensus view on many of the internal relationships of Insecta and their wider grouping, the Hexapoda. These are discussed below.

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