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Over recent years, researchers have developed the scenario of fleas feasting on pterosaurs and feathered dinosaurs during the Jurassic Period of Earth history. Fossils of strashilid insects have been suggested to have piercing and sucking mouthparts and hind pincers suitable for attaching to a host's hairs or feathers. Further reading on this interpretation is to be found in Rasnitsyn (1992) and Gao et al. (2012). Other putative flea species have also been found and Poinar's (2012) perspective paper has the title: "the 165-million-year itch" (see also here).
A new interpretation of the strashilids has now been proposed after examining spectacular recent finds from the Daohugou beds, Inner Mongolia. The researchers have concluded that the fossils are the remains of flies, not fleas. They were not ectoparasites living on animals, but highly specialized flies adapted to aquatic environments.
"Although definitive Middle Jurassic ectoparasites (fleas) on vertebrates were reported recently from the Daohugou outcrops, earlier speculation regarding strashilids as terrestrial ectoparasites can be rejected owing to an alternative hypothesis stemming from the discovery of hitherto unknown large membranous wings and females of Strashila, in two cases preserved in copula. Males and females have identical head morphologies but differ strongly in the absence of the hind leg pincers in females, which excludes ectoparasitism on terrestrial vertebrates as these are reinterpreted as sex-related structures for grasping the female rather than a host's integument. In addition, unlike ectoparasitic insect lineages, the body is cylindrical in strashilids, rather than dorsoventrally or laterally flattened." (page 94, Huang et al. 2013)
From Poinar (2012): "(A) Reconstruction of the 150 million year old Jurassic flea-like Strashila incredibilis. Note the incredibly long and powerful hind legs clasping the base of two feathers on a feathered dinosaur and a well-developed proboscis. Strashila may have fed on pterosaurs or feathered dinosaurs. Drawing by the author. (B ) Reconstruction of the Chinese Jurassic pseudoflea, Pseudopulex jurassicus, imagined parasitizing a feathered dinosaur." (Source here)
It is worth noting that our understanding of strashilids has changed as a result of finding well preserved specimens. Previous work was based on meagre fossil evidence and was inherently speculative. It is possible to generalise the lessons that should be learned from this: when fossil evidence is sparse, it is unwise to speculate. Unfortunately, the pressures to publicise finds are substantial and are unlikely to diminish. The media is always looking for stories that create interest, and speculation appears to be preferred above caution.
It is also worth noting that the new interpretation of strashilids locates them in a more familiar context. The researchers note characters that are "virtually identical with those of the recent nematoceran flies", are "comparable in critical details with those of nymphomyiids", and "similar to those of nymphomyiids". "We conclude that the Strashilidae were a highly specialized dipteran clade related to Nymphomyiidae, with an aquatic or amphibious life history." This paragraph summarises the evidence:
"These structures provide evidence that strashilids share several potential apomorphies with modern nymphomyiid flies, including the finer details of the antennae, ocelli, wings, terminalia and legs. Their reduced mouthparts and wings are inefficient for feeding and active flight, respectively, and indicate an ephemeral life history, akin to modern nymphomyiid taxa. Similar to modern nymphomyiids, adult Strashilidae probably mated soon after emergence, shed their wings, crawled beneath the water, mated, and in some cases died in copula, with the superposing male grasping the female below, thereby explaining the discoveries of females grasped by males, and both without their wings." (page 96)
It is not without significance that the initial interpretation of the fossil strashilids considered them to be primitive flea-like insects. There were hints from some characters that made sense in the perspective provided by evolutionary theory. However, what we can now see is a highly specialized insect related to Nymphomyiidae. It is derived rather than primitive. And this specialised insect, similar to modern nymphomyiids, was part of the Jurassic world. Is it possible that the Darwinian mindset colours the interpretations placed on fossilised species? Before this thought is dismissed out of hand, the case of the strashilids serves as a salutary case study to ponder.
Amphibious flies and paedomorphism in the Jurassic period
Diying Huang, Andre Nel, Chenyang Cai, Qibin Lin & Michael S. Engel
Nature, 495, 94-97 (07 March 2013) | doi:10.1038/nature11898
Abstract: The species of the Strashilidae (strashilids) have been the most perplexing of fossil insects from the Jurassic period of Russia and China. They have been widely considered to be ectoparasites of pterosaurs or feathered dinosaurs, based on the putative presence of piercing and sucking mouthparts and hind tibio-basitarsal pincers purportedly used to fix onto the host's hairs or feathers. Both the supposed host and parasite occur in the Daohugou beds from the Middle Jurassic epoch of China (approximately 165 million years ago). Here we analyse the morphology of strashilids from the Daohugou beds, and reach markedly different conclusions; namely that strashilids are highly specialized flies (Diptera) bearing large membranous wings, with substantial sexual dimorphism of the hind legs and abdominal extensions. The idea that they belong to an extinct order is unsupported, and the lineage can be placed within the true flies. In terms of major morphological and inferred behavioural features, strashilids resemble the recent (extant) and relict members of the aquatic fly family Nymphomyiidae. Their ontogeny are distinguished by the persistence in adult males of larval abdominal respiratory gills, representing a unique case of paedomorphism among endopterygote insects. Adult strashilids were probably aquatic or amphibious, shedding their wings after emergence and mating in the water.