Comparative morphological and functional analysis of wing coupling and related structures in caddisflies (Insecta: Trichoptera)

Several orders of morphologically four-winged insects have evolved mechanisms that enforce a union between the mesothoracic and metathoracic wings (forewings and hindwings) during the wing beat cycle. Such mechanisms result in a morphologically tetrapterous insect flying as if it were functionally dipterous, and these mechanisms have been described for several insect orders. The caddisfly suborders Annulipalpia and Integripalpia (Trichoptera) have each evolved wing coupling apparati. At least three systems have evolved within the suborder Annulipalpia (Polycentropodidae; Hydropsychidae: Hydropsychinae, Macronematinae), and within Integripalpia the evolution of wing coupling mechanisms is diverse to the point that it defies simple enumeration into discrete categories; conservatively seven different mechanisms have evolved. The comparative and inferred functional morphology of the putative wing coupling mechanisms is described for families in both Annulipalpia and Integripalpia. A novel form-functional complex putatively involved with at-rest forewing-forewing coupling is described for Hydropsychidae: Smicrideinae (Annulipalpia) and Philorheithridae (Integripalpia: Brevitentoria), and the form-functional consequences of this novel mechanism for forewing-hindwing coupling are elaborated. Experimental evidence regarding coupled versus uncoupled flight is given for both Annulipalpia and Integripalpia, including the discovery of wing coupling in Polycentropodidae (Annulipalpia) and Brachycentridae (Integripalpia: Plenitentoria). Based on experimental evidence and comparative morphological considerations it is argued that wing coupling is a far more complex phenomenon than presence- absence. It is likely that all Trichoptera have coupled wings on the downstroke, with the synchronous downstroke enforced either by a fully developed wing coupling apparatus or, when present, by the interaction of the forewing jugal lobe and the hindwing frenular-type setae. It is also suggested that for several families and sub-family clades, the morphology of the wing coupling apparatus is a synapomorphy and contributes characters for phylogenetic analyses