Timema
Encyclopedia
Timema is a genus
of relatively short-bodied, stout stick insects native to the far western United States
. The genus was first described in 1895 by Samuel Hubbard Scudder
, based on observations of the species Timema californicum.
Compared to other stick insects (order Phasmatodea), the genus Timema is considered basal
; that is, the earliest "branch" to diverge from the phylogenetic tree that includes all Phasmatodea. To emphasize this outgroup status, all stick insects not included in Timema are sometimes described as "Euphasmatodea."
Five of the twenty-one species of Timema are parthenogenetic
, including two species that have not engaged in sexual reproduction for one million years, the longest known asexual period for any insect
.
in that their tarsi
have three segments rather than five. For stick insects, they have relatively small, stout bodies, so that they look somewhat like earwigs (order Dermaptera).
.
In 2008, researchers studying the presence or absence of a dorsal stripe suggested that it has independently evolved several times in Timema species and is an adaptation for crypsis on needle-like leaves. All of the eight Timema species with a dorsal stripe have at least one host plant with needle-like foliage. Of the thirteen unstriped species, seven feed only on broadleaf plants. Four (T. ritensis, T. podura, T. genevieve, and T. coffmani) rest during the day on the host plant's trunk rather than its leaves and have bodies that are brown, gray, or tan. Only two species (T. nakipa and T. boharti) have green unstriped morphs that feed on needle-like foliage; both are generalist feeders that also feed on broadleaf hosts.
The species Timema cristinae exhibits both striped and unstriped populations depending on the host plant, a form of polymorphism
that clearly illustrates the camouflage function of the stripe. The earliest ancestors of this species were generalists
that fed on plants belonging to both the genera Adenostoma
and Ceanothus
. They eventually diverged into two distinct ecotype
s with a more specialist
host plant preference. One ecotype prefers to feed on Adenostoma while the other ecotype prefers to feed on Ceanothus. The Adenostoma ecotype possesses a white dorsal stripe, an adaptation to blend in with the needle-like leaves of the plant, while the Ceanothus ecotype does not (Ceanothus spp. have broad leaves). The Adenostoma ecotype is also smaller, with a wider head, and shorter legs.
These characteristics are genetically inherited and has been interpreted as the early stages of the speciation
process. The two ecotypes will eventually become separate species once reproductive isolation
is achieved. At the moment, both ecotypes are still capable of interbreeding and producing viable offspring, as such they are still considered a single species.
) through which the nymph will emerge. Timema females use particles of dirt, which they have previously ingested, to coat their eggs.
The eggs of many stick insects, including Timema, are attractive to ants, who carry them away to their burrows to feed on the egg's capitulum
, while leaving the rest of the egg intact to hatch. The emerging nymph passes through six or seven instars before reaching adulthood.
Timema males, in sexual species of Timema, show a consistent pattern of courting behavior. The male climbs onto the back of the female and, after a short display of vibrating and waving, they proceed to mate. (Rejection by the female is possible but uncommon.) The male then rides on the female's back for up to five days, a behavior often referred to as "guarding" the female.
Several species of Timema are parthenogenetic
: that is, females can reproduce asexually, producing viable eggs without male participation.
According to Tanja Schwander of Simon Fraser University
, "Timema are indeed the oldest insects for which there is good evidence that they have been asexual for long periods of time." She heads a team of researchers who found that five Timema species (T. douglasi, T. monikensis, T. shepardi, T. tahoe and T. genevieve) have used only asexual reproduction
for more than 500,000 years, with T. tahoe and T. genevieve reproducing asexually for over one million years.
Host plants of the different Timema species include Pseudotsuga menziesii (Douglas fir), Sequoia sempervirens (Californian redwood), Arctostaphylos
spp. (manzanita), Ceanothus
spp., Adenostoma fasciculatum
(chamise), Abies concolor (white fir), Quercus spp. (oak), Heteromeles arbutifolia (toyon), Cercocarpus spp. (mountain-mahogany), Eriogonum
sp. (buckwheat), and Juniperus spp. (juniper).
(female only).
Timematidae and the suborder Timematodea. Their clade
is considered considered basal
to the order Phasmatodea; that is, many scientists believe that Timema-type stick insects represent the earliest "branch" to diverge from the phylogenetic tree that gave rise to all the stick insects of Phasmatodea. This primal distinction is referenced by the name "Euphasmatodea", which is given to all the clades of Phasmatodea other than the suborder Timematodea.
Twenty-one species have been described; in addition there is at least one undescribed species known to exist:
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of relatively short-bodied, stout stick insects native to the far western United States
Western United States
.The Western United States, commonly referred to as the American West or simply "the West," traditionally refers to the region comprising the westernmost states of the United States. Because the U.S. expanded westward after its founding, the meaning of the West has evolved over time...
. The genus was first described in 1895 by Samuel Hubbard Scudder
Samuel Hubbard Scudder
Samuel Hubbard Scudder was an American entomologist and palaeontologist.Born in Boston, Massachusetts, Scudder may be most widely known for his essay on the importance of first-hand, careful observation in the natural sciences...
, based on observations of the species Timema californicum.
Compared to other stick insects (order Phasmatodea), the genus Timema is considered basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
; that is, the earliest "branch" to diverge from the phylogenetic tree that includes all Phasmatodea. To emphasize this outgroup status, all stick insects not included in Timema are sometimes described as "Euphasmatodea."
Five of the twenty-one species of Timema are parthenogenetic
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
, including two species that have not engaged in sexual reproduction for one million years, the longest known asexual period for any insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
.
Description
Timema spp. differ from other PhasmatodeaPhasmatodea
The Phasmatodea are an order of insects, whose members are variously known as stick insects , walking sticks or stick-bugs , phasmids, ghost insects and leaf insects...
in that their tarsi
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...
have three segments rather than five. For stick insects, they have relatively small, stout bodies, so that they look somewhat like earwigs (order Dermaptera).
Cryptic coloration and camouflage
Timema walking sticks are night-feeders who spend daytime resting on the leaves or bark of the plants they feed on. Timema colors (primarily green, gray, or brown) and patterns (which may be stripes, scales, or dots) match their typical background, a form of crypsisCrypsis
In ecology, crypsis is the ability of an organism to avoid observation or detection by other organisms. It may be either a predation strategy or an antipredator adaptation, and methods include camouflage, nocturnality, subterranean lifestyle, transparency, and mimicry...
.
In 2008, researchers studying the presence or absence of a dorsal stripe suggested that it has independently evolved several times in Timema species and is an adaptation for crypsis on needle-like leaves. All of the eight Timema species with a dorsal stripe have at least one host plant with needle-like foliage. Of the thirteen unstriped species, seven feed only on broadleaf plants. Four (T. ritensis, T. podura, T. genevieve, and T. coffmani) rest during the day on the host plant's trunk rather than its leaves and have bodies that are brown, gray, or tan. Only two species (T. nakipa and T. boharti) have green unstriped morphs that feed on needle-like foliage; both are generalist feeders that also feed on broadleaf hosts.
The species Timema cristinae exhibits both striped and unstriped populations depending on the host plant, a form of polymorphism
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
that clearly illustrates the camouflage function of the stripe. The earliest ancestors of this species were generalists
Generalist and specialist species
A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources . A specialist species can only thrive in a narrow range of environmental conditions or has a limited diet. Most organisms do not all fit neatly into either...
that fed on plants belonging to both the genera Adenostoma
Adenostoma
Adenostoma is a genus of shrubs containing only two species: Adenostoma fasciculatum , and Adenostoma sparsifolium . Both are native to coastal California and Baja California, while A...
and Ceanothus
Ceanothus
Ceanothus L. is a genus of about 50–60 species of shrubs or small trees in the buckthorn family Rhamnaceae. The genus is confined to North America, the center of its distribution in California, with some species in the eastern United States and southeast Canada, and others extending as far south...
. They eventually diverged into two distinct ecotype
Ecotype
In evolutionary ecology, an ecotype,Greek: οίκος = home and τύπος = type, coined by Göte Turesson in 1922 sometimes called ecospecies, describes a genetically distinct geographic variety, population or race within species , which is adapted to specific environmental conditions.Typically, ecotypes...
s with a more specialist
Generalist and specialist species
A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources . A specialist species can only thrive in a narrow range of environmental conditions or has a limited diet. Most organisms do not all fit neatly into either...
host plant preference. One ecotype prefers to feed on Adenostoma while the other ecotype prefers to feed on Ceanothus. The Adenostoma ecotype possesses a white dorsal stripe, an adaptation to blend in with the needle-like leaves of the plant, while the Ceanothus ecotype does not (Ceanothus spp. have broad leaves). The Adenostoma ecotype is also smaller, with a wider head, and shorter legs.
These characteristics are genetically inherited and has been interpreted as the early stages of the speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
process. The two ecotypes will eventually become separate species once reproductive isolation
Reproductive isolation
The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...
is achieved. At the moment, both ecotypes are still capable of interbreeding and producing viable offspring, as such they are still considered a single species.
Life cycle and reproduction
Timema eggs are soft, ellipsoidal, and about two mm long, with a lid-like structure at one end (the operculumOperculum (animal)
An operculum is an anatomical feature, a stiff structure resembling a lid or a small door that opens and closes, and thus controls contact between the outside world and an internal part of an animal...
) through which the nymph will emerge. Timema females use particles of dirt, which they have previously ingested, to coat their eggs.
The eggs of many stick insects, including Timema, are attractive to ants, who carry them away to their burrows to feed on the egg's capitulum
Capitulum
The term Capitulum can refer to several things:*In botany, a type of flower head where the bracts are located under the basis, such as a daisy's*Also in botany, the top of a Sphagnum moss plant with compact clusters of young branches....
, while leaving the rest of the egg intact to hatch. The emerging nymph passes through six or seven instars before reaching adulthood.
Timema males, in sexual species of Timema, show a consistent pattern of courting behavior. The male climbs onto the back of the female and, after a short display of vibrating and waving, they proceed to mate. (Rejection by the female is possible but uncommon.) The male then rides on the female's back for up to five days, a behavior often referred to as "guarding" the female.
Several species of Timema are parthenogenetic
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
: that is, females can reproduce asexually, producing viable eggs without male participation.
According to Tanja Schwander of Simon Fraser University
Simon Fraser University
Simon Fraser University is a Canadian public research university in British Columbia with its main campus on Burnaby Mountain in Burnaby, and satellite campuses in Vancouver and Surrey. The main campus in Burnaby, located from downtown Vancouver, was established in 1965 and has more than 34,000...
, "Timema are indeed the oldest insects for which there is good evidence that they have been asexual for long periods of time." She heads a team of researchers who found that five Timema species (T. douglasi, T. monikensis, T. shepardi, T. tahoe and T. genevieve) have used only asexual reproduction
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
for more than 500,000 years, with T. tahoe and T. genevieve reproducing asexually for over one million years.
Habitat
The geographic range of Timema is limited to mountainous regions of western North America between 30° and 42° N. They are found primarily in California, as well as in a few other neighboring states (Oregon, Nevada, Arizona) and in northern Mexico. All are herbivores, primarily feeding on host plants found in chapparal.Host plants of the different Timema species include Pseudotsuga menziesii (Douglas fir), Sequoia sempervirens (Californian redwood), Arctostaphylos
Arctostaphylos
Arctostaphylos is a genus of plants comprised by the manzanitas and bearberries. They are shrubs or small trees.There are about 60 species of Arctostaphylos, ranging from ground-hugging arctic, coastal, and mountain species to small trees up to 6 m tall. Most are evergreen , with small oval...
spp. (manzanita), Ceanothus
Ceanothus
Ceanothus L. is a genus of about 50–60 species of shrubs or small trees in the buckthorn family Rhamnaceae. The genus is confined to North America, the center of its distribution in California, with some species in the eastern United States and southeast Canada, and others extending as far south...
spp., Adenostoma fasciculatum
Adenostoma fasciculatum
Adenostoma fasciculatum is a flowering plant native to California and northern Baja California. This shrub is one of the most widespread plants of the chaparral biome.-Description:...
(chamise), Abies concolor (white fir), Quercus spp. (oak), Heteromeles arbutifolia (toyon), Cercocarpus spp. (mountain-mahogany), Eriogonum
Eriogonum
Eriogonum is the scientific name for a genus of flowering plants, in the family Polygonaceae. The genus is found in North America and is known as wild buckwheat. This is a highly species-rich genus, and indications are that active speciation is continuing...
sp. (buckwheat), and Juniperus spp. (juniper).
Phylogeny
General phylogenetic relationships within Timema (Law & Crespi, 2002). Species marked with ♀ are parthenogeneticParthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
(female only).
Classification
Timema is the only member of the familyFamily (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Timematidae and the suborder Timematodea. Their clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
is considered considered basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
to the order Phasmatodea; that is, many scientists believe that Timema-type stick insects represent the earliest "branch" to diverge from the phylogenetic tree that gave rise to all the stick insects of Phasmatodea. This primal distinction is referenced by the name "Euphasmatodea", which is given to all the clades of Phasmatodea other than the suborder Timematodea.
Twenty-one species have been described; in addition there is at least one undescribed species known to exist:
- Timema bartmani
- Timema boharti
- Timema californicum
- Timema chumash
- Timema coffmani
- Timema cristinae
- Timema dorotheae
- Timema douglasiTimema douglasiTimema douglasi is a stick insect native to northern California and southern Oregon. It was first identified in 1996 as a specialist feeder on old-growth Douglas fir. It is one of five parthenogenetic species of Timema....
- Timema genevieve
- Timema knulli
- Timema landelsensis
- Timema monikensis
- Timema morongensis
- Timema nakipa
- Timema nevadense
- Timema petita
- Timema podura
- Timema poppensis
- Timema ritensis
- Timema shepardi
- Timema tahoe
- Timema sp. nov. on limber pineLimber PineThe Limber Pine, Pinus flexilis, is a species of pine tree-the family Pinaceae that occurs in the mountains of the Western United States, Mexico, and Canada. It is also called Southwestern White Pine and Rocky Mountain White Pine...
See also
- ThelytokyThelytokyThelytoky comes from the Greek thely, meaning "female", and tokos, meaning "birth". Thelytokous parthenogenesis is a type of parthenogenesis in which females are produced from unfertilized eggs. It is rare in the animal kingdom and has only been reported in about 1500 species...
- Sexual dimorphismSexual dimorphismSexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
- Evolutionary arms raceEvolutionary arms raceIn evolutionary biology, an evolutionary arms race is an evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other, resembling an arms race, which are also examples of positive feedback...
- Sexual conflictSexual conflictSexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly the mode and frequency of mating, leading to an evolutionary arms race between males and females. The conflict encompasses the actions and behaviors of both sexes to influence...
- Cnemidophorus neomexicanusCnemidophorus neomexicanusThe New Mexico whiptail is a species of lizard found in the southern United States in New Mexico and Arizona, and in northern Mexico in Chihuahua. It is the official state reptile of New Mexico...