The insect order Phasmatodea is known for large slender insects masquerading as twigs or bark. In contrast to these so-called stick insects, the subordinated clade of leaf insects (Phylliidae) are dorso-ventrally flattened and therefore resemble leaves in a unique way. Here we show that the origin of extant leaf insects lies in the Australasian/Pacific region with subsequent dispersal westwards to mainland Asia and colonisation of most Southeast Asian landmasses. We further hypothesise that the clade originated in the Early Eocene after the emergence of angiosperm-dominated rainforests. The genus Phyllium to which most of the ~100 described species pertain is recovered as paraphyletic and its three non-nominate subgenera are recovered as distinct, monophyletic groups and are consequently elevated to genus rank. This first phylogeny covering all major phylliid groups provides the basis for future studies on their taxonomy and a framework to unveil more of their cryptic and underestimated diversity.

While the leaf insects (Phylliidae) are a well-supported group within Phasmatodea, the genus Phyllium Illiger, 1798 has repeatedly been recovered as paraphyletic. Here, the Phyllium (Phyllium) celebicum species group is reviewed and its distinctiveness from the remaining Phylliini genera and subgenera in a phylogenetic context based on morphological review and a phylogenetic analysis of three genes (nuclear gene 28S and mitochondrial genes COI and 16S) from most known and multiple undescribed species is shown. A new genus, Cryptophylliumgen. nov., is erected to partially accommodate the former members of the celebicum species group. Two species, Phyllium ericoriaiHennemann et al., 2009 and Phyllium bonifacioi Lit & Eusebio, 2014 morphologically and molecularly do not fall within this clade and are therefore left within Phyllium (Phyllium). The transfer of the remaining celebicum group members from Phyllium Illiger, 1798 to this new genus creates the following new combinations; Cryptophyllium athanysus (Westwood, 1859), comb. nov.; Cryptophyllium celebicum (de Haan, 1842), comb. nov.; Cryptophyllium chrisangi (Seow-Choen, 2017), comb. nov.; Cryptophyllium drunganum (Yang, 1995), comb. nov.; Cryptophyllium oyae (Cumming & Le Tirant, 2020), comb. nov.; Cryptophyllium parum (Liu, 1993), comb. nov.; Cryptophyllium rarum (Liu, 1993), comb. nov.; Cryptophyllium tibetense (Liu, 1993), comb. nov.; Cryptophyllium westwoodii (Wood-Mason, 1875), comb. nov.; Cryptophyllium yapicum (Cumming & Teemsma, 2018), comb. nov.; and Cryptophyllium yunnanense (Liu, 1993), comb. nov. The review of specimens belonging to this clade also revealed 13 undescribed species, which are described within as: Cryptophyllium animatumgen. et sp. nov. from Vietnam: Quang Nam Province; Cryptophyllium bankoigen. et sp. nov. from Vietnam: Quang Ngai, Thua Thien Hue, Da Nang, Gia Lai, Quang Nam, and Dak Nong Provinces; Cryptophyllium bollensigen. et sp. nov. from Vietnam: Ninh Thuan Province; Cryptophyllium daparogen. et sp. nov. from China: Yunnan Province; Cryptophyllium echidnagen. et sp. nov. from Indonesia: Wangi-wangi Island; Cryptophyllium faulknerigen. et sp. nov. from Vietnam: Quang Ngai and Lam Dong Provinces; Cryptophyllium icarusgen. et sp. nov. from Vietnam: Lam Dong and Dak Lak Provinces; Cryptophyllium khmergen. et sp. nov. from Cambodia: Koh Kong and Siem Reap Provinces; Cryptophyllium limogesigen. et sp. nov. from Vietnam: Lam Dong, Dak Lak, and Dak Nong Provinces; Cryptophyllium liyananaegen. et sp. nov. from China: Guangxi Province; Cryptophyllium nuichuaensegen. et sp. nov. from Vietnam: Ninh Thuan Province; Cryptophyllium phamigen. et sp. nov. from Vietnam: Dong Nai and Ninh Thuan Provinces; and Cryptophyllium wennaegen. et sp. nov. from China: Yunnan Province. All newly described species are morphologically described, illustrated, and molecularly compared to congenerics. With the molecular results revealing cryptic taxa, it was found necessary for Cryptophyllium westwoodii (Wood-Mason, 1875), comb. nov. to have a neotype specimen designated to allow accurate differentiation from congenerics. To conclude, male and female dichotomous keys to species for the Cryptophylliumgen. nov. are presented.

After successful laboratory rearing of both males and females from a single clutch of eggs, the genus Nanophyllium Redtenbacher, 1906 (described only from males) and the frondosum species group within Phyllium (Pulchriphyllium) Griffini, 1898 (described only from females) are found to be the opposite sexes of the same genus. This rearing observation finally elucidates the relationship of these two small body sized leaf insect groups which, for more than a century, have never been linked before. This paper synonymizes the frondosum species group with Nanophyllium Redtenbacher, 1906 in order to create a singular and clearly defined taxonomic group. Five species are transferred from the Phyllium (Pulchriphyllium) frondosum species group and create the following new combinations: Nanophyllium asekiense (Größer, 2002), comb. nov.; Nanophyllium chitoniscoides (Größer, 1992), comb. nov.; Nanophyllium frondosum (Redtenbacher, 1906), comb. nov.; Nanophyllium keyicum (Karny, 1914), comb. nov.; Nanophyllium suzukii (Größer, 2008), comb. nov. The only taxon from this species group not transferred from the frondosum species group to Nanophyllium is Phyllium (Pulchriphyllium) groesseri Zompro, 1998. Based on protibial exterior lobes, this species belongs in the schultzei species group as described in Hennemann et al. 2009 and is therefore excluded from further discussion here. The rearing of Nanophyllium also yielded the male Nanophyllium asekiense (Größer, 2002), comb. nov. thus, enabling comparison of this male to the other previously known Nanophyllium species. Two new species of nano-leaf insects are described within, Nanophyllium miyashitaisp. nov., from Morobe Province, Papua New Guinea, and Nanophyllium daphnesp. nov., from Biak Island, Papua Province, Indonesia. With such distinct sexual dimorphism in Nanophyllium between sexes, which have only now been matched up via captive rearing, illustrated within are numerous specimens which might represent the unknown opposite sexes of the many currently known species of Nanophyllium. Due to pronounced sexual dimorphism in Nanophyllium, only future captive rearing or molecular analysis will match up the many unknown sexes. To conclude, with the description of two new Nanophyllium species, dichotomous keys to species for known males and females are presented.

Within the last two years, the leaf insects of the genus Phyllium of both the islands of Java and Sumatra have been reviewed extensively based on morphological observations. However, cryptic species which cannot be differentiated morphologically may be present among the various populations. Since it has frequently been demonstrated that analyses based on molecular data can bring clarity in such cases, we conducted a phylogenetic analysis based on three genes (nuclear gene 28S and mitochondrial genes COI and 16S) from the Phyllium species of these islands. The results show distinct molecular divergence for several populations and suggest the presence of two new cryptic species, morphologically inseparable from Phyllium hausleithneri Brock, 1999. From Sumatra, the population originally thought to be a range expansion for Phyllium hausleithneri, is now here described as Phyllium nisussp. nov., with the only consistent morphological difference being the color of the eggs between the two populations (dark brown in P. hausleithneri and tan in P. nisussp. nov.). Further, an additional population with purple coxae from Java was morphologically examined and found to have no consistent features to separate it morphologically from the other purple coxae species. This cryptic species from Java was however shown to be molecularly distinct from the other purple coxae populations from Sumatra and Peninsular Malaysia and is here described as Phyllium gardabagusisp. nov. In addition, Phyllium giganteum is here officially reported from Java for the first time based on both historic and modern records of male specimens.

With every molecular review involving Chitoniscus Stål, 1875 sensu lato samples from Fiji and New Caledonia revealing polyphyly, the morphology from these two distinct clades was extensively reviewed. Morphological results agree with all previously published molecular studies and therefore Trolicaphylliumgen. nov. is erected to accommodate the former Chitoniscus sensu lato species restricted to New Caledonia, leaving the type species Chitoniscus lobiventris (Blanchard, 1853) and all other Fijian species within Chitoniscus sensu stricto. Erection of this new genus for the New Caledonian species warrants the following new combinations: Trolicaphyllium brachysoma (Sharp, 1898), comb. nov., Trolicaphyllium erosus (Redtenbachher, 1906), comb. nov., and Trolicaphyllium sarrameaense (Größer, 2008a), comb. nov. Morphological details of the female, male, freshly hatched nymph, and egg are illustrated and discussed alongside the Chitoniscus sensu stricto in order to differentiate these two clades which have been mistaken as one for decades.