of the insects. Two of the most important insect orders, Lepidoptera (butterflies and moths) and Diptera (true flies), along with Trichoptera (caddisflies), probably evolved from ancestors belonging to, or strictly related to, the Mecoptera. Evidence includes anatomical and biochemical similarities as well as transitional fossils, such as Permotanyderus and Choristotanyderus, which lie between the Mecoptera and Diptera. The group was once much more widespread and diverse than it is now, with four suborders during the Mesozoic. It is unclear as of 2020 whether the Mecoptera form a single clade, or whether the Siphonaptera (fleas) are inside that clade, so that the traditional "Mecoptera" taxon is paraphyletic. However the earlier suggestion that the Siphonaptera are sister to the Boreidae is not supported; instead, there is the possibility that they are sister to another Mecopteran family, the Nannochoristidae. The two possible trees are shown below: (a) Mecoptera (clades in boldface) is paraphyletic, containing Siphonaptera: (b) Mecoptera is monophyletic, sister to Siphonaptera: Internal relationships All the families were formerly treated as part of a single order, Mecoptera. The relationships between the families are, however, a matter of debate. The cladogram, from Cracraft and Donoghue 2004, places the Nannochoristidae as a separate order, with the Boreidae, as the sister group to the Siphonaptera, also as its own order. The Eomeropidae is suggested to be the sister group to the rest of the Mecoptera, with the position of the Bittacidae unclear. Of those other families, the Meropeidae is the most basal, and the relationships of the rest are not completely clear. Biology Morphology Mecoptera are small to medium-sized insects with long beaklike rostra, membranous wings and slender, elongated bodies. They have relatively simple mouthparts, with a long labium, long mandibles and fleshy palps, which resemble those of the more primitive true flies. Like many other insects, they possess compound eyes on the sides of their heads, and three ocelli on the top. The antennae are filiform (thread-shaped) and contain multiple segments. The fore and hind wings are similar in shape, being long and narrow, with numerous cross-veins, and somewhat resembling those of primitive insects such as mayflies. A few genera, however, have reduced wings, or have lost them altogether. The abdomen is cylindrical with eleven segments, the first of which is fused to the metathorax. The cerci consist of one or two segments. The abdomen typically curves upwards in the male, superficially resembling the tail