BR. J. ENT. NAT. HIST., 7: 1994 99 



SEPARATION OF SOME ERISTA US SPECIES USING ABDOM1N \l 



COLOUR PATTERN 



Graham J. Hoi.i.oway 



Department of Pure & Applied Zoology, University of Reading, Whiteknights, P.O. Box 228, 



Reading, Berkshire RG6 2AJ. 



Some sets of hoverfly species are rather difficult or laborious to separate in the 

 field (e.g. Baccha spp., Paragus spp., Sphaerophoria spp.) and collectors often decide 

 to capture a few to identify them later on with the aid of a microscope. When all 

 species concerned are interesting for one reason or another this poses no real problem, 

 but occasionally a rare or scarce hoverfly resembles a particularly abundant species. 

 In this situation many records of the less common species may be completely 

 overlooked and, consequently, an accurate picture of their distribution and abundance 

 may take a very long time to emerge. Any studies on long-term population changes 

 in a species such as this would have little value given the unreliability of past 

 distribution and abundance estimates. 



Stubbs and Falk (1983) describe Ehstalis abusivus Collin as a "local" species but 

 "the commonest Eristalis in some coastal districts". The separation of E. abusivus 

 from the closely related E. arbustorum (L.) is relatively straightforward, but requires 

 inspection of the fine structure of the arista. Other characters that can be used include 

 the distance over which the eyes touch in the males (van der Goot, 1981) and the 

 amount of yellow on the tibia of the middle leg. Apart from perhaps the eye character 

 in the males, none of them are very accessible to use in the field. However, the biggest 

 problem is that similar species, such as E. arbustorum and E. nemorum (L.), are 

 so abundant and widespread. In a mass of Eristalis species, not many entomologists 

 would be prepared to devote time to checking hundreds of individuals on the chance 

 that a few E. abusivus are present. Stubbs and Falk (1983) also consider it likely that 

 this species is overlooked in the field. 



Recently, I carried out a study of colour variation in certain Eristalis species using 

 museum specimens held at the Natuurhistorische Museum in Leiden, The Netherlands 

 (Holloway, 1993). The sample sizes were large and for E. arbustorum, E. abusivus 

 and E. nemorum 3169, 843 and 826 individuals were inspected, respectively. Using 

 these specimens, I was able to ascertain not only the amount of pattern variation 

 shown, but also any consistent pattern differences among the species (Figure 1). 

 Although this type of quantitative variation is not generally considered useful to 

 identify species, I found, in the course of my study, that I was able to identify main 

 individuals immediately solely on the basis of their colour pattern. A couple of 

 E. arbustorum that had somehow crept into the E. abusivus boxes stuck out like sore 

 thumbs! It occurred to me that colour pattern differences may be a quick and easy 

 way of provisionally assessing in the field the occurrence of E. abusivus. Having 

 captured a likely looking candidate, the accepted qualitative characters could then 

 be used to confirm identification. 



There was always a considerable difference between the sexes in all species with 

 most of the variation in females occurring on tergite 2 and in males on tergite 3. There 

 were a number of important consistent differences between the colour patterns ol 

 E. arbustorum and E. abusivus. In E. abusivus, the yellow patches on the abdomen 

 never touched the trailing edge of tergite 2 in females and tergite 3 in males. In 

 E. arbustorum, the trailing edges of these tergites were often reached by the yellow 

 patches. In female E. abusivus, the yellow patches on tergite 2 when present assumed 

 a hooked shape, whilst in female E. arbustorum the yellow patches were more 



