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Tetrophthalmi is an extinct suborder of Opiliones (commonly known as harvestmen or daddy-longlegs) that had both median and lateral eyes. First described in 2014, it is known from two extinct species. Phylogenetic analysis suggests that this eye arrangement is the ancestral condition for harvestmen, placing Tetrophthalmi and Cyphophthalmi in a basal position within Opiliones.
In 2024, a study discovered that living members of Opiliones have retained lateral eyes, as well as a second pair of vestigial median eyes. A reanalysis of harvestman phylogeny based on 78 genes and an updated understanding of morphology revealed that Tetrophthalmi are derived members of the living suborder Eupnoi (true daddy-longlegs).
The CT scan revealed that H. argus had two sets of eyes – one set mounted on the ocularium in the middle of head and one set on its side on the ozopore. Performing comparative analysis on 158 morphological features, Garwood's team determined H. argus was closely related to Eophalangium sheari. The two species were not, however, as closely related to the 270 other species of harvestmen studied. The team determined H. argus and E. sheari shared three features – two pairs of eyes, an open gonostome (a part of the reproductive system), and external male genitalia – which distinguish them from all other harvestmen. Thus, the two species were classified into a new suborder, Tetrophthalmi.
Hastocularis argus and the new suborder Tetrophthalmi help explain how arachnid eyes developed. The classification also helps clarify when side-eyed and fore-head eyed harvestmen split into distinct .
Genetic analysis performed on a modern Phalangium opilio specimen found a suppressed gene that, if active, would generate a second pair of eyes at the lateral location, providing independent evidence that four eyes is the ancestral condition. Garwood et al. also argue that a Carboniferous harvestmen diversification is more consistent with changes observed in other terrestrial , which have been linked to high oxygen levels during that period.
Another recent phylogeny recovered Hastocularis as an early branching opilionid in the harvestman tree, along with the Cyphophthalmi. Eophalangium, the other genus in Garwood et al.'s Tetrophthalmi was recovered in a polytomy with the remaining oplionids, however.
The most recent phylogenetic analyses of Opiliones phylogeny have recovered Tetrophthalmi as derived members within the suborder Eupnoi, which has resulted in older ages of Opiliones diversification than previously estimated. The restoration of Eophangium sheari, the oldest known Opiliones fossil, to membership in Eupnoi is consistent with the original description of this species as a true daddy-longlegs.
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