Citrine Forktail (Ischnura hastata)
• Scientific name: Ischnura hastata (Say, 1839)
• English name: Citrine Forktail
• German name: Rätselhafte Pechlibelle
• French name: N/A
• Dutch name: Geel lantaarntje
• Swedish name: N/A
• Norwegian name: N/A
• Finnish name: N/A
• Danish name: N/A
• Italian name: N/A
• Czech name: N/A
• Slovenian name: N/A
• Bulgarian name: N/A
• Typical length: 20-27 mm
• Hindwing: 11-15 mm
The tiny Citrine Forktail (Ischnura hastata) is an American species which is distributed widely from southern Canada, throughout the USA, and into Mexico and Central and South-America. Its presence in Europe is limited to an all-female population on the Portuguese islands of the Azores, some 1,400 kilometres off the Portuguese west coast. It is the smallest damselfly species in the USA (and in Europe), measuring only 20-27 mm.
Immature females are a bright orange colour with black on the head, the top of the thorax, and abdominal segments 6-8. As they mature, the black colouration on the abdomen increases and the orange colour is gradually replaced by (an olivaceous) green or grey. Old females develop a light pruinescence on the thorax and abdomen. Unusually for an Ischnura species, there are no androchrome females.
Males have a greenish thorax and bright yellow abdomen, again with black markings on the head, thorax and abdominal segments 1-7. The last three segments are completely yellow (hence the 'citrine' in the species' common name) and S10 carries a prominent upright projection, which is notched (hence the 'forktail' and indeed the specific name 'hastata', meaning 'armed with a spear').
Apart from its diminutive size, there are two aspects which render Ischnura hastata unique among the world's odonates:
- The male's pterostigma in the front wing is not directly connected to the costa (i.e. the wing's leading edge), but is separated from and connected to it by a light-coloured petiole. Where the pterostigma 'ought to be', the colour of the costa itself is a whitish yellow (rather than black). No other known odonate species shares this characteristic of an in-wing pterostigma. The forewing pterostigma is a bright orange-red colour and is considerably larger than the grey-black pterostigma in the hindwing.
- The aforementioned all-female population on the Azores is unique in that it reproduces parthenogenetically. The hypothesis that this might be the case was first postulated in 1990 when attempts to collect males from the islands proved unsuccessful (as they have remained since). In recent years the Azores population has been the subject of intensive study (notably by Adolfo Cordero and Olalla Lorenzo), and proof of the females' ability to reproduce parthenogenetically was provided by Cordero in his laboratory in Vigo, Spain, when he managed to raise several consecutive generations (of females only, of course) from Azorean larvae. When and why the islands' population has become parthenogenetic is not clear. First discovered (though initially misidentified) in 1938, it is now believed that the population has been present on the islands for at least a few centuries (but not millennia), and was initially transported to the islands from America on strong westerly winds. It is unknown whether the initial population consisted of males and females but something then started to push the population towards parthenogenesis or whether only parthenogenetic females colonized the islands. As a phenomenon, parthenogenesis is not all that uncommon among insects and other animals, and in some cases can be explained in terms of bacterial infections (e.g. by the Wolbachia bacterium). But in the case of Ischnura hastata no evidence of bacterial infection has been found among the Azorean females. Whether today's parthenogenesis in this population is the result of some other, as yet unknown environmental factor which over time eliminated the males from the population, or whether the females themselves decided they might as well do without the males, will probably remain a matter of speculation for some time yet. What is clear, though, is that even if only a small part of the females in a population start to develop a tendency towards asexual, parthenogenetic reproduction, then ultimately that parthenogenesis could become widespread among the population, with fewer and fewer females being available to males and, consequently, fewer and fewer male offspring being produced.