"Beware the Jabberwock, my son!
The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
The frumious Bandersnatch! "
"Jabberwocky", by Lewis Carroll (1832-1898)
Insects are the most successful multicellular animals on earth today and there is no reason to assume that in the geological past this situation was very different. Some groups have also successfully adapted to blood-sucking habits.
Blood-sucking insects include various groups. One of the largest is the group of the "flies" (Diptera), more specific the mosquitoes (Culicidae), the snipe flies (Rhagionidae), the athericid flies (Athericidae), the blackflies (Simuliidae), the biting midges (Ceratopogonids), the sand flies (Phlebotominae) and the horse flies (Tabanidae) as most important and well-known examples.
Fleas and lice are the second largest group of obligatory blood feeders. Also some bugs of the Heteroptera have evolved a taste for blood.
Even if there is no direct evidence that the ancestors of these insects feed on dinosaurs, like a paleo-flea fossilised on a dinosaur, the contemporaneity of these groups and dinosaurs makes it highly probable that the insects did benefit from the presence of such large animals.
Imprints of dinosaur-skin show that some species were covered with scutes or scales, other species possessed scales or tubercles embedded probably in a thin skin and finally some dinosaurs were covered with bristles, filaments, proto-feathers and feathers. In every case these various types of skin did not provided always an impenetrable wall against all sorts of blood-sucking organisms.
Not all insects that bite need blood to feed themselves, as some species use blood only in certain growth phases or for egg production. Some insects are generalists and will attack all sorts of cold-blooded or warm-blooded animals, like fishes, amphibians, reptiles, birds and mammals. Other are more specialized and prefer single groups or specific species.
Insects, despite the common claim, don't bite, but scratch or cut the skin with specific structures of their mouthparts. Mosquitoes then directly punctuate a blood vessel and suck the blood from it (so called capillary-feeders). Other insects - like blackflies, biting midges, sand flies and horse flies - lacerate the blood vessels and then lick the blood that accumulates in the wound (pool-feeders).
Mosquitoes (Culicidae) are the most familiar group of the blood-sucking insect, however they are quite rare in the fossil record and only single specimens confirm their presence during the Cretaceous. Most of the recent forms are opportunists, as they will attack whatever host is available, even if they prefer mammals and birds.
Biting midges (Ceratopogonidae) resemble a tiny version of mosquitoes. This group is commonly found in Cretaceous amber and it is probable that they in the past also targeted dinosaurs. Ceratopogonids today are pool-feeders, they will attack all sorts of vertebrates and concentrate their effort on regions of the body were blood vessels are easily accessible, like the area surrounding the eye or joints, were scales are smaller or the skin thinner.
Blackflies (Simuliidae) are tiny insects like the biting midges. Legends know that large swarms are capable to weakening and even kill animals due the suffered loss of blood.
Fossil remains are known from the Jurassic and Cretaceous in Europe, Australia, Asia, and North America. Modern forms are not known to feed on reptiles, but they feed on birds.
Sand flies (Phlebotominae) are probably one of the earliest groups of flies in which some species evolved the ability to suck blood. The primary herbivorous group, feeding on plant sap, evolved during the Cretaceous the habit to feed on animal wounds and later acquired the ability to actively suck blood. Most of them are general feeders, feeding on all sorts of vertebrates. However some species prefer lizards and snakes and have no problem to reach the soft skin under the overlapping scales.
Horse flies and deer flies of the family Tabanidae were widespread throughout the Cretaceous and are today strong, persistent flyers and pool-feeders on both warm- and cold-blooded animals.
Their "bite" is nasty and often where the insect feed a painful wheal develops. Today at least four species of horseflies are known to prey on crocodiles and anacondas in the Amazon, also turtles and birds can not escape their attacks.
Fleas (Siphonaptera) and lice (Phthiraptera) today thrive under the protection of hairs and feathers on both mammals and bird and it seems reasonable to assume that they or similar organisms could also accept feathered dinosaurs as hosts.
Fleas or flea-like insects, like Strashila incredibilis, have been described from Mesozoic sediments. Strashila incredibilis is characterized by large claws and well developed hind legs, used maybe to improve the grip on feathers or bristles.
Another Mesozoic flea-like insect (it is tentatively attributed to Mecoptera, a group possibly related to true fleas) is the species Saurophthirus longipes. The prolonged proboscis, the large claws and long legs have been interpretated as modifications for parasitic behaviour on pterosaur wings or to grasp the borders of large dinosaur scales.
The oldest true fleas seem to be the new described specimens from the Jurassic of China. It is interesting to note that the mouthparts of these fleas are exceptionally well developed, maybe to be able to deal with the resistant skin of reptiles and dinosaurs. In case of "reptilian" skinned dinosaurs however it must be noted that modern fleas try to avoid reptiles as hosts, probably due the lack of shelter on the scaly skin.
Lice are highly specialized parasites of mammals and birds and are subdivided in two groups based on their feeding behaviour: the biting (Mallophaga) and sucking (Anoplura) type. The second group feeds exclusively on mammalian blood, while the Mallophaga feed on feathers, hair, skin and blood of birds.
Fossil lice are rare findings. One of the most strange fossils is the species Saurodectes , described from the early Cretaceous of Siberia and supposedly identified as large (14mm) lice living on dinosaurs or pterosaurs.
BREHM, A.E. (1892): Insekten, Tausendfüßer und Spinnen. Brehms Tierleben Bd.9. Ernst Ludwig Taschenberg (all images taken from this book, images in public domain)
GRIMALDI, D. A. & ENGEL, M. S. (2005): Evolution of the Insects. Cambridge University Press: 755
POINAR, G. & POINAR, R. (2007): What Bugged the Dinosaurs? Insects, Disease, and Death in the Cretaceous. Princeton University Press: 296
RASNITSYN, A.P. & QUICKE, D.L.J. (eds.) (2002): History of insects. Kluwer Academic Press: 517