**2. Sexual dimorphism in Coleoptera**

Outstanding structural differences between males and females of many species of beetles have been described by naturalist and scientists during the last 200 years. Most part of such

© 2013 Romero-López and Morón, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

differences is located on the head and prothorax of males, such as in the form and size of the mandibles, length of antennal or palpi segments, horn-like structures on the frons or on the pronotal surface. Other differences are found in the size of whole body, development of elytra and hind wings, as well as in the length of fore and hind legs, or in the shape of pygidial plate. Less frequent is the strong difference in color or body vestiture. Flightless females are commonly found in species of many families, but larval-like females are the rule into the species of Phengodidae [1,2,3,4]. Sexual dimorphism is more common in the species of the suborder Polyphaga than in the suborder Adephaga, and appears to be a recent feature, because few fossil specimens of diverse families dated from Pliocene and Pleistocene shown dimorphic structures [5,6,7].

Sexual Dimorphism in Antennae of Mexican Species of *Phyllophaga* 

Rutelinae and Dynastinae. Some species of these subfamilies are famous by it striking sexual differences. First example, each antenna of the male of *Polyphylla petiti* Guerín (Melolonthinae) is formed by seven enlarged, flattened antennomeres that are five times longer than the remainder three basal antennomeres, meanwhile each antenna of the female is formed by five briefly elongated antennomeres that are as long as the remainder five antennomeres. It is evident that the much expanded olfactory surface of the male is developed in correspondence with the perception of sex compounds produced by the

Second example, metasternum of the male of *Chrysina macropus* Francillon (Rutelinae) strongly produced, hind coxa is wide, hind femur is swollen and provided with a strong spine, and the tibia is enlarged and curved with setae on the inner border (Figures 1 A-B), in the female the metasternum is flattened, hind coxa and femur are much narrower than in male and hind tibia is short and nearly straight, without setae on the inner border. Looks like the male embraces female with his large hind legs during coupling, but really these legs are not used in this form. It is much possible that are useful during combat with other cospecific males as forceps formed between femur and tibia, that may produce much force derived from the increased musculature inside the metathorax, coxa and femur; such

forceps also may be used as defense against big predators [Morón unpublished data].

cause of the acquisition of fantastic forms."

Third example, horn-like projections on the head and pronotum of *Dynastes hercules* L. (Dynastinae) may be as long as body length or even longer that this. Both projections may act as long forceps, applying force derived from cervical muscles to head projection. Such force is sufficient to cut or broken the elytra of other male of "Hercules beetle", but is useful to embrace it a turnout of the tree branches where they inhabit, after fighting during some minutes. But this conclusion needed many years to mature, because much speculation surrounded the observation of dead specimens in collections, and studies of live beetles in nature or in captivity were scarce and incomplete during near a century. After a long discussion on the dimorphic structures in horned beetles, with arguments and data of other authors, including Darwin, [14] concluded that "The horns of a beetle, the size of which is increasing gradually as generations succeed one another, will as a result become more and more disproportionate in size, regardless of the fact that they may be quite useless, and the absence of the restraining and modeling influence of natural selection will be a contributory

A large number of observations on captive Hercules beetles in Venezuela support the following comments: "The sequence of the beetle encounter is unvarying. The two meet head on, and the projecting horns touch and click, spread wide and close, the whole object of the opening phase being to get a grip outside the opponent's horns. When the four horns are closed together, there is a deadlock. All force in now given to pinching, with the apparent desire to crush and injure some part of head or thorax. Again and again, both opponents back away, freeing their weapons, and then rush in for a fresh grip. Once this hold is attained and a firm grip secured, the beetle rears up and up to an unbelievably vertical stance. At the zenith of this pose it rests upon the tip of the abdomen and the tarsi of

female [13].

(Coleoptera: Scarabaeoidea: Melolonthidae) 19
