**3. Sexual dimorphism in Coleoptera Scarabaeoidea**

"A most remarkable distinction between the sexes of many beetles is presented by the great horns which rise from the head, thorax and clypeus of the males... These horns, in the great family of Lamellicorns, resemble those of various quadrupeds, such as stags, rhinoceroses, etc. and are wonderful both from their size and diversified shapes. … The females generally exhibit rudiments of the horns in the form of small knobs or ridges; but some are destitute of even a rudiment…. The extraordinary size of the horns, and their widely different structure in closely-allied forms, indicate that they have been formed for some important purpose; but their excessive variability in the males of the same species leads to the inference that this purpose cannot be of a definite nature. … The most obvious conjecture is that they are used by the males for fighting together; but they never had been observed to fight…" [8].

Members of the superfamily Scarabaeoidea (Lamellicornia and Pectinicornia) exhibit ones of the most striking dimorphic structures of the beetle species. Cephalic horn-like projections are present in the males of species from the subfamilies Dynastinae, Rutelinae, Cetoniinae, Scarabaeinae, and Geotrupinae. Much enlarged mandibles are common in the males of the subfamilies Lucaninae, Dorcinae, Lampriminae, Chiasognathinae and Odontolabinae. Males of Melolonthinae, Rutelinae and Pleocomidae frequently show the segments of the antennal club enlarged and expanded. Pronotal horn-like or tooth-like projections are developed in the males of the subfamilies Dynastinae, Cetoniinae, Scarabaeinae, Geotrupinae and Orphninae. Much enlarged fore legs (femur, tibia, tarsus) are characteristic in males of the subfamily Euchirinae, meanwhile males of the genus *Pachylomera* (Scarabaeinae) have fore femora much swell. Enlarged hind legs (femur and tibia) are observed in males of some genera of Rutelinae and Scarabaeinae. Females apterous or with short hind wings have been reported in nearly all the subfamilies of Scarabaeoidea, but the female of *Pachypus candidae* Petagna (Pachypodinae) lacks also the elytra. Except for horn-like structures, usually, males have smaller and slender bodies than females, but females of the family Pleocomidae are much larger and stouter than males [9,10].

### **3.1. Dimorphism and behavior in Melolonthidae (chafers, rhinoceros beetles)**

Following the classification of [11] updated by [12], the family Melolonthidae ("melolonthids") is formed by the world wide members of Melolonthinae (sensu lato), 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 female [13].

18 Sexual Dimorphism

Pleistocene shown dimorphic structures [5,6,7].

much larger and stouter than males [9,10].

**3. Sexual dimorphism in Coleoptera Scarabaeoidea** 

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

"A most remarkable distinction between the sexes of many beetles is presented by the great horns which rise from the head, thorax and clypeus of the males... These horns, in the great family of Lamellicorns, resemble those of various quadrupeds, such as stags, rhinoceroses, etc. and are wonderful both from their size and diversified shapes. … The females generally exhibit rudiments of the horns in the form of small knobs or ridges; but some are destitute of even a rudiment…. The extraordinary size of the horns, and their widely different structure in closely-allied forms, indicate that they have been formed for some important purpose; but their excessive variability in the males of the same species leads to the inference that this purpose cannot be of a definite nature. … The most obvious conjecture is that they are used

by the males for fighting together; but they never had been observed to fight…" [8].

Members of the superfamily Scarabaeoidea (Lamellicornia and Pectinicornia) exhibit ones of the most striking dimorphic structures of the beetle species. Cephalic horn-like projections are present in the males of species from the subfamilies Dynastinae, Rutelinae, Cetoniinae, Scarabaeinae, and Geotrupinae. Much enlarged mandibles are common in the males of the subfamilies Lucaninae, Dorcinae, Lampriminae, Chiasognathinae and Odontolabinae. Males of Melolonthinae, Rutelinae and Pleocomidae frequently show the segments of the antennal club enlarged and expanded. Pronotal horn-like or tooth-like projections are developed in the males of the subfamilies Dynastinae, Cetoniinae, Scarabaeinae, Geotrupinae and Orphninae. Much enlarged fore legs (femur, tibia, tarsus) are characteristic in males of the subfamily Euchirinae, meanwhile males of the genus *Pachylomera* (Scarabaeinae) have fore femora much swell. Enlarged hind legs (femur and tibia) are observed in males of some genera of Rutelinae and Scarabaeinae. Females apterous or with short hind wings have been reported in nearly all the subfamilies of Scarabaeoidea, but the female of *Pachypus candidae* Petagna (Pachypodinae) lacks also the elytra. Except for horn-like structures, usually, males have smaller and slender bodies than females, but females of the family Pleocomidae are

**3.1. Dimorphism and behavior in Melolonthidae (chafers, rhinoceros beetles)** 

Following the classification of [11] updated by [12], the family Melolonthidae ("melolonthids") is formed by the world wide members of Melolonthinae (sensu lato), 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].

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 cause of the acquisition of fantastic forms."

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

Sexual Dimorphism in Antennae of Mexican Species of *Phyllophaga* 

developed on the last sternites and pygidial plate [18]. Many of these are frequently applied as taxonomic characters, but it function remain generalized. Few studies on the allometric growth of hypertelic structures in species with strong sexual dimorphism had been

Scarce research had been carried on the microstructures, mainly on those related with sensorial functions, as the diverse types of sensilla placed on the lamellar surfaces of antennomeres such as was detailed by [21]. These structures were briefly studied in some Mexican species by [18,22,23,24]. The structure, distribution and function of the antennal sensilla reveal much interest, because represent the main way to understand the prereproductive behavior, and the evolution of the chemical communication in this group of

**4. Studies of sexual dimorphism in antennae of Mexican species of** 

**4.1. Methodology for the study of antennae of Mexican melolonthids** 

After taxonomical identification using the keys proposed by Morón [18], females and males of *P. obsoleta*, *P. ravida* and *Phyllophaga opaca* Moser were randomly chosen for length measurement with the Image Tool 3.0 software program [28]. Length was measured in each specimen from the clypeus to the pygidium. The head of each previously measured specimen was separated from the body and preserved in 70% ethanol. The antennae of females and males of each species were separated and measurements of total length, width, and area of each lamella, were obtained with the Image Tool 3.0 software program. Afterwards, the three lamellae forming each antennal club were separated, labeled, and grouped according to sex and lamellar side (internal or external surfaces). The lamella located farther away from the head was denominated distal lamella (DL), while the nearest was called proximal lamella (PL) and the one

*4.1.1. Measurement of body, antennae and lamellae dimensions* 

between these two, middle lamella (ML).

Sexual chemical communication in some Melolonthidae involves the production and release of specific chemicals by the emitter and the detection and olfactory processing of these signals leading to appropriate behavioral responses in the receiver [25]. Chemicals released from melolonthid females are captured in the sensilla located on both sides of male antennal lamellae [21,18,26]. The genus *Phyllophaga* is formed by more than 800 species, but only in *Phyllophaga anxia* LeConte [27] and in the antennae of Mexican species *Phyllophaga obsoleta* Blanchard [23] and *Phyllophaga ravida* Blanchard [24] have the different types of sensilla been studied. In the following sections data from *P. obsoleta* and *P. ravida* are remembered and compared with the data of another species of *Phyllophaga* distributed

published in Mexico [19,20].

insects.

**P***hyllophaga*

in Mexico.

(Coleoptera: Scarabaeoidea: Melolonthidae) 21

**Figure 1.** *Chrysina macropus* Francillon. (A) Male, ventral view, showing the hypertrophied hind legs. (B) Female, ventral view. Scale lines 5 mm. Photos M. A. Morón.

the hind legs, the remaining four legs outstretched in mid air, and the opponent held sideways, kicking impotently… after 2-8 seconds the victim is slammed down or is carried away … before to banging to earth will take place" [15]. Other academics discussed on the essential components of such phenomenon or on details of the above described behavior, as [16] who comments: "The display of species showing extreme intrasexual selection function both to attract females to intimidate other males. Precopulatory displays are short or absent. The male Hercules beetle, for example, evidently engages in none whatever. Occasionally he picks a female up and carries her aimlessly about for a short while, but the significance of the behavior is unknown. During both transportation and copulation the female remains outwardly passive". With elegance and precision, [17] objectively wrote: "The only function for beetle horns which has been confirmed by detailed observations is that of weapons for use in intraspecific fights. Many horn designs remain to be studied… More data are needed to answer outstanding questions about the significance of multiple horn designs and the selective pressures favoring fighting in some species. Several factors may have predisposed beetles to evolve horns more readily than other insects".

#### **3.2. Study of dimorphism in melolonthid beetles in Mexico**

The males of nearly all the species of Melolonthidae usually present a number of small structural differences more or less directly associated with the search of females and coupling behavior. Most of these modifications are located in the legs, as tarsal pads, specially curved tarsal claws, tibial spurs, rows of tarsal setae, etc. Other modifications are developed on the last sternites and pygidial plate [18]. Many of these are frequently applied as taxonomic characters, but it function remain generalized. Few studies on the allometric growth of hypertelic structures in species with strong sexual dimorphism had been published in Mexico [19,20].

20 Sexual Dimorphism

**Figure 1.** *Chrysina macropus* Francillon. (A) Male, ventral view, showing the hypertrophied hind legs.

the hind legs, the remaining four legs outstretched in mid air, and the opponent held sideways, kicking impotently… after 2-8 seconds the victim is slammed down or is carried away … before to banging to earth will take place" [15]. Other academics discussed on the essential components of such phenomenon or on details of the above described behavior, as [16] who comments: "The display of species showing extreme intrasexual selection function both to attract females to intimidate other males. Precopulatory displays are short or absent. The male Hercules beetle, for example, evidently engages in none whatever. Occasionally he picks a female up and carries her aimlessly about for a short while, but the significance of the behavior is unknown. During both transportation and copulation the female remains outwardly passive". With elegance and precision, [17] objectively wrote: "The only function for beetle horns which has been confirmed by detailed observations is that of weapons for use in intraspecific fights. Many horn designs remain to be studied… More data are needed to answer outstanding questions about the significance of multiple horn designs and the selective pressures favoring fighting in some species. Several factors may have predisposed

The males of nearly all the species of Melolonthidae usually present a number of small structural differences more or less directly associated with the search of females and coupling behavior. Most of these modifications are located in the legs, as tarsal pads, specially curved tarsal claws, tibial spurs, rows of tarsal setae, etc. Other modifications are

(B) Female, ventral view. Scale lines 5 mm. Photos M. A. Morón.

beetles to evolve horns more readily than other insects".

**3.2. Study of dimorphism in melolonthid beetles in Mexico** 

Scarce research had been carried on the microstructures, mainly on those related with sensorial functions, as the diverse types of sensilla placed on the lamellar surfaces of antennomeres such as was detailed by [21]. These structures were briefly studied in some Mexican species by [18,22,23,24]. The structure, distribution and function of the antennal sensilla reveal much interest, because represent the main way to understand the prereproductive behavior, and the evolution of the chemical communication in this group of insects.
