FNZ 12 - Pompilidae (Insecta: Hymenoptera) - Geographical Variation
Harris, AC 1987. Pompilidae (Insecta: Hymenoptera). Fauna of New Zealand 12, 160 pages.
( ISSN 0111-5383 (print), ; no. 12. ISBN 0-477-02501-3 (print), ). Published 13 Nov 1987
Segments of antennae and palpi progressively shorten and thicken with increasing latitude in some species (e.g., Priocnemis (Trichocurgus) monachus, P. (T.) conformis).
P. (T.) carbonarius and P. (T.) crawi n.sp. both have distinctive populations in Otago in which the size of the malar space and the dorsal teeth on the hind tibial comb rows of females are significantly different.
Teratological abnormalities affecting the body, appendages, and wing venation are surprisingly frequent, and occur in all species.
Parallel colour variation
Four endemic species of Pompilidae show striking parallel clinal variations in the colour of the integument throughout New Zealand. In the north, both males and females of Priocnemis (T.) conformis, Sphictostethus fugax, and S. calvus n.sp. are bright red. In the upper half of the South Island the head and thorax may be black, but the metasoma is always bright red; south of latitude 44°S the entire body is usually shining jet black. In a fourth species, Epipompilus insularis, colour variation is restricted to females because sexual dimorphism is unusually marked, males always having the body and appendages black and the wings hyaline. Although colour variation of females parallels the three species listed above, it is less spectacular. Melanism increases regularly southwards from North Cape (34°25'S) to southern Stewart Island (47°16'S). It occurs as black spots on otherwise red sclerites; there is very seldom a gradual increase in overall tone. Harris (1974) measured the degree of melanism of 2513 adult wasps, and showed that the parallel patterns of increasing melanism are strongly correlated with increasing latitude, cloudiness, and summer precipitation and with decreasing insolation and temperature. Moreover, specimens captured early in the season (e.g., September-November) are slightly darker than those taken late in the season (e.g., February-May).
Harris (1974) reared pupae at different temperatures. Pupae of Sphictostethus calvus n.sp. and S. fugax kept at 5°C produced black-bodied adults. Red-bodied adults emerged from pupae kept at 20°C regardless of the coloration of the parents. The degree of melanism in S. fugax and S. calvus was shown to be directly related to the temperature regime experienced by the pupa, whereas in Priocnemis (T.) conformis and Epipompilus insularis a genetic factor is involved also. Melanism could not be induced experimentally in S. nitidus (other than on the mesosoma, beneath the golden pile).
In summarising melanism in the New Zealand Pompilidae, Harris (1974) made the following points.
- Four species - Sphictostethus fugax, S. calius, Priocnemis (T.) conformis (both sexes), and Epipompilus insularis (females only) - show gradual and parallel increases in melanism southwards from North Cape to southern Stewart Island.
- This corresponds very closely to a number ot climatic factors, of which southward decline in temperature and insolation and increase in cloudiness are the most significant.
- Pompilidae are generally active only in direct sunlight.
- Black adults (S. fugax, S. calvus, P. conformis) were shown to warm up more rapidly and attain higher internal temperatures, higher metabolic rates, and greater activity when exposed to sunlight than red individuals of the same species from the same locality. Preliminary tests suggested that the range of internal temperatures supporting voluntary activity is the same for black and red individuals of the same species (Harris 1974).
- The degree of melanism of the adult probably adapts it to the range of temperature and amount of insolation characteristic of a given locality (latitude and altitude).
- The critical period for determining the degree of melanism of the adult occurs during pupal development in spring, after the prepupal moult, when pigment deposition in the cuticle begins.
- It was shown experimentally that the amount of black pigment in S. fugax and S. calvus is directly related to temperature. For instance, offspring of both black and red females reared at pupal temperatures of 24°C were uniformly red, whereas those reared at 5°C were uniformly black. Intermediate pupal temperatures produced adults with predictable proportions of black and red. Experiments on P. (T.) conformis and E. insularis (females), on the other hand, revealed a greater genetic influence on the occurrence of melanism (Harris 1974).
- The experimental results (see (7), above) correlate with field conditions. In southern populations, female E. insularis legs and P. conformis bodies are uniformly black, unlike those of S. fugax and S. calvus, in which occasional red-bodied forms occur depending on local microclimates. Dunedin provides good examples of microclimatically induced colour variation in S. fugax and S. calvus. Both have the abdomen black north of Dunedin on Leith Saddle, which is frequently shrouded in orographic mist. At a comparable altitude on southeastern Mt Maungatua (south of Dunedin) - which usually escapes orographic mist on days when Leith Saddle is clouded, as well as having higher spring air and ground temperatures - abdomens of S. fugax and S. calvus are completely red. The Otago Peninsula is sometimes misty, and there both species have the abdomen mostly red but striped with broad bands of black.
- Apart from colour-variable areas of mesosomal integument which are concealed beneath golden pile, S. nitidus remains uniformly red-bodied throughout its range, showed no increased cuticular melanism on experimental chilling other than on the mesosoma, and (significantly) does not occur in the far south.
Geographically separated colour morphs
Geographical barriers of the past (e.g., Pliocene Auckland Straits, Pleistocene glacial mid Canterbury between the contemporary Rangitata River and Waitaki River) and present (e.g., Cook Strait, Southern Alps) are marked by abrupt disjunctions between adjoining populations of the same species, or by hybrid-zones (Harris 1974). These disjunctions are at the root of many of the taxonomic problems. For example, several geographical isolates have some of the characteristics of subspecies, or even full species. I have adopted a lumping approach to their taxonomy, and regard them as polytypic species. For instance, Sphictostethus nitidus has three clear-cut forms, separated respectively by Cook Strait, where there is a complete disjunction, and by the former Auckland Straits, where there is a narrow hybrid zone. In P. conformis, distribution patterns and relative numbers of red and black forms, together with (unpublished) data from limited electrophoretic tests, suggest that populations in the mid Canterbury hybrid-zone are not in panmixis. In this roughly 65-km-long band P. conformis populations occur in which roughly 47% of individuals have black abdomens, 47% have red abdomens, and the remainder form a complete gradation between the extremes. Within the hybrid-zone there are many areas in which the two colour morphs occur together (e.g., Mills Bush, Peel Forest). In other places the colour morphs are separated both spatially and temporally. In several valleys every individual on one side is black (usually a northeast-facing slope that receives only morning sunlight), whereas the opposite side supports wasps with red abdomens (e.g., at Kelsey's Bush, Waimate).
North of the hybrid-zone all individuals of P. conformis have red abdomens and south of it the abdomen is invariably black. Harris (1974) suggested that pre-Pleistocene populations throughout New Zealand formed north-south clines related directly to climate, similar to those of contemporary populations of S. fugax and S. calvus. A Pleistocene ice cap in the Mt Cook region extending westwards to the sea via piedmont glaciers, combined with periglacial conditions on the east, divided P. conformis into two allopatric populations. These would have converged during interglacials.
In New Zealand today insolation is greatest in Marlborough and least in Southland (the approximate positions of Pleistocene refugia), and Harris (1974) suggested that under Pleistocene conditions similar to today's but more extreme, melanism may have become partially genetically fixed in the southern population during one (or more) glacial advances, so that in some interglacials (including the present one) populations in the area of convergence showed many of the signs of secondary intergradation. The situation can just as well be regarded as a zone of balanced polymorphism between two roughly monomorphic populations with, overlain, an environmentally controlled variation of phenotype. Hence, I have regarded the name diligens given by Smith (1876) to the melanic form as a junior synonym of conformis, which has page priority in the same paper. (The holotype of diligens, moreover, is an incompletely melanic form from Peel Forest, at the northern limit of the hybrid-zone.)
Notwithstanding this, overall melanism in P. conformis increases fairly regularly southwards throughout its range.
Sphictostethus nitidus comprises three races. A South Island form is separated by Cook Strait from a southern and central North Island form, and this forms a cline between Auckland and Warkworth with a Northland form. The Southern Alps separate Westland and East Coast forms of S. fugax. In both species, geographical forms are distinguishable mainly by differences in wing coloration.
Characteristic forewing patterns of the colour-variable species are shown in Figures 220 - 236.