| Amara ovata and similata | ||||||||
| Before discussing the differences between these species we have given some
guidance that will allow them to be separated from the other Amara species occurring in Britain; it
hardly seems sensible trying to separate the pair if they cannot be recognised in the first place.
We have done this at the generic level because, in the case of these and similar Amara species,
they will be recognised immediately and cannot be mistaken for any other genus of carabids. What follows takes
the form of a key and as such will be interesting to those just beginning to get interested in beetles but
it is only really intended to isolate our two species. It will also, accidentally and fortuitously, adequately
isolate A plebeja (Gyllenhal), which is featured.
For the moment it deals only with those species possessing a puncture at the base of the scutellary stria but
we hope eventually to expand it to include other species. |
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| 1 | Elytra with a pore/puncture at the base of the abbreviated scutellary stria. | 2 | ||||||
| -- | Elytra without such a pore. | Other Amara species | ||||||
| 2 | Terminal spur of pro-tibia trifid. | (subgenus Zezea csiki) 3 | ||||||
| -- | Terminal spur of pro-tibia simple. | 4 | ||||||
| 3 | Length of beetle less than 8mm. | plebeja (Gyllenhal) | ||||||
| -- | Length of beetle at least 8mm. | strenua Zimmermann | ||||||
| 4 | Antennae entirely pale, pronotal base coarsely punctured. | praetermissa (Sahlberg, C.R.) | ||||||
| -- | Antennae darkened from third or fourth segment. | 5 | ||||||
| 5 | Less than 7mm. | anthobia Villa & Villa | ||||||
| -- | More than 7mm. | 6 | ||||||
| 6 | Setiferous puncture inside pronotal hind angles close to lateral margin; separation about twice the puncture diameter. | 7 | ||||||
| -- | Setiferous puncture inside pronotal hind angles more removed from lateral margin; separation at least three times the puncture diameter. | 8 | ||||||
| 7 | Amara ovata and A. similata. | see below. | ||||||
| 8 | Tibiae pale. Pronotal front angles rounded and hardly projecting, pronotal base punctuate and with evident fovea. | nitida Sturm | ||||||
| -- | Tibia very dark. Pronotal front angles sharp and protruding, pronotal base impunctate and fovea obsolete. | montivaga Sturm |
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| Amara ovata and
Amara similata are undoubtedly superficially very similar and there is some,
occasionally wide, variation in the characters used to separate them so that some specimens will not be readily
assigned. On the other hand both are reasonably common and so for the purpose of recording the species it should
not be too difficult to obtain specimens which can be identified with confidence.
Joy summed up the differences and also provided line drawings to show the overall form of each; these look like they should be diagnostic but this will soon change when trying to compare real specimens, worse still the fact that without experience these sort of things become all the more ambiguous the longer one looks at them. Nonetheless it is worth giving Joy’s separation as, overall, it works and anyway there appear to have been no more external morphological characters used since Joy that make the matter any easier: |
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| 15 (16) | Form broader; tibiae black or pitchy; basal foveae of thorax unpunctured. Length 9-11mm | ovata Fabricius | ||||||
| 16 (15) | Form narrower; tibiae red; basal fovea of thorax finely punctured. Length 8-10mm | similata Gyllenhal | ||||||
| Given reference material or (perhaps) photographs of each species the difference in ‘form’ may (or,
admittedly, may not) be appreciated. The generally light tibiae of similata are distinctive but these may
vary; Luff (2007) writes ‘often mid to dark brown’ and for ovata ‘legs black’. Lindroth (1974) states
similarly for ovata: ‘Tibiae black or piceous, and for similata ‘tibiae paler, often rufous’.
Ostensibly a good character and one that has worked for us in every case. Punctuation of the pronotal base is
likewise a good character; mentioned by Joy but not explained and so, without experience, may cause more problems
than it solves, more especially so when faced with a specimen with some very sparse and fine puncturation present
only around the edges of the obscurely delimited basal fovea, this may raise the question of whether Joy was
referring to this or to something more substantial. Lindroth adds to the lack of confidence by stating ‘base
punctate’ for similata but then ‘base impunctate or almost so’ for ovata! In all the specimens we
have examined the pronotal base in similata is finely or very finely punctured around or within the inner
fovea or depression. In ovata it is not punctured.
Shape. Lindroth gives a line drawing of the outline of the left side of the pronotum of each species and, were this invariably the case, separation would be straightforward but in a series of specimens this character varies, or seems to, and depends crucially upon the angle from which it is viewed; it needs to be looked at directly from above and this will usually involve manipulating the specimen. Briefly, in similata the basal half of the lateral edge is straightened when compared with the rather strongly curved apical half; in ovata the lateral edge is evenly curved from front to hind angles. In one of our specimens of ovata (kindly verified by M. Telfer) the basal half of the lateral margin is straight as in similata. Size. This is variously quoted as a little larger for similata e.g. Joy (above), Lindroth or Forsythe who give 7.8-10mm for similata and 8-9.5mm for ovata. In the latest British key Luff gives 8-9.5mm for both species and so this is best ignored as a diagnostic aid. In summary the best characters remain those used by Joy but it needs to be understood that e.g. leg colour varies a little, pronotal shape likewise and perhaps more than just a little, the pronotum in ovata may be slightly and very finely punctured and the overall form, given some importance by Joy, will probably (as with many groups) become obvious only with experience, trying to appreciate this too soon and after having looked at only a few specimens will probably cause only frustration. As a last resort there are differences in the aedeagus and these we hope to feature below soon but for the purpose of recording a local fauna there should be more than enough typical specimens to look at. |
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