**1. Introduction**

16 Sexual Dimorphism

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Sexual dimorphism in body, antennae, lamellae and chemosensilla types of some Mexican species of *Phyllophaga* are recorded. In *Phyllophaga obsoleta* and *Phyllophaga ravida* the male's body is slightly larger than the female's, and its antennae and lamellae are longer than the females. Meanwhile, in *Phyllophaga opaca* the body and its antennae and lamellae of the males are very similar in size than the females. The external morphology of sensilla on the antennae has been described using scanning electron microscopy. The antennal club of these beetles consists of three terminal plates: proximal, middle, and distal lamellae. In all these species, the main sensilla types were identified on the internal and external surfaces of lamellae from both sexes: placodea (PLAS), auricilica (AUS), basiconica (BAS), coeloconica (COS), trichodea (TRS), and chaetica (CHS). The first four types have been considered as chemosensilla and the last two as mechanoreceptor sensilla. For *P. opaca*, fifteen types of chemosensilla were found: four types of PLAS (I, II, III and VIII), four types of AUS (I, II, III and IV), five types of BAS (I, II, III, IV and V), and two types of COS (I and III). This is very similar to that observed in *P. ravida* and therefore it is suggested that these data can relate to the taxonomy of the genus *Phyllophaga*, since both species belong to the same subgenus. To give continuity to this type of comparative studies with other Mexican species of Melolonthidae to complement the information on the phylogeny of the group, is necessary in addition to their chemical communication, information about their sexual dimorphism phocused on antennal micro-morphology, genital structure, and reproductive behavior.
