Clever Caterpillars: The Awkward Journey to Adulthood Matt Przylucki
Kingdom
Animalia
Phylum
Arthropoda
Class
Insecta
Order
Lepidoptera
Suborder
Glossata
Infraorder
Heteroneura
Division
Ditrysia
Superfamily
Zygaenoidea
Family
Limacodidae
Abstract: Limacodidae, also know as slug caterpillars, are some of the most diverse
insects on the planet.In larvae form they are dorso‐ventrally compressed, have reduced abdominal prolegs that act as suckers, underdeveloped thoracic legs, and a retractable head. Their sleek and stealthy design is often compared to a slug, which suggests the origin of their common name.This unique family of caterpillars has
evolved many different defense mechanisms, which has allowed them to thrive in highly competitive communities. Several species are equipped with stinging spines, or setae, that contain a powerful poison. (Murphy, 2010)Once contact has been initiated the fragile spines often stick into the intruder causing an irritating nettling sensation.These adaptations suggest the strong influence predators have played in the evolution of the family Limacodidae.Once they begin their transformation to the adult stage they begin to construct their silk cocoon, which becomes hardened and round.(Mullen, 1993) This provides optimal protection during the final molt. Many species retain their distasteful poison in their adult moth stage, but lose their extraordinary color and become rather drab in color. Slug caterpillars are often found in tropical and subtropical climates, but there are 50 known species present in North America.(Mullen, 1993) This particular family of caterpillars has undergone astonishing feats in evolutionary terms and displays the most unusual morphologies when compared to other caterpillars. The most common Limacodidae in North America is Sibinestimulea, or the saddleback caterpillar.They can be seen on a variety of different plants including a wide range of fruit trees. With their advanced defense mechanisms they generally have high survival rates.Limacodidae have been known to frighten even the most intimidating predatorssuch as paper wasps and assassin bugs. In this paper we will explore the various adaptations that allow slug caterpillars to flourish in areas of high predation.
Introduction:
Sir Joseph Banks, an English botanist, was the first to record an encounter
with a slug caterpillar while visiting Australia. In 1770 while on James Cook’s first voyage he was stung by one of the many species present in Australia. Since then these particular caterpillars have been of special interest to scientists because of their incredible adaptations. Foraging Behaviors:
Nearly all caterpillars found throughout the world are herbivorous and feed
on a variety of vegetation. This vegetation is often abundant depending on their habitat and allows them to flourish in many different environments.Limacodidae can be found on nearly every continent and often display colors unique to their preferred delicacy. This morphology is a special type of camouflage known as cryptic coloration and is key to their survival.(Evans and Schmidt, 1990)
Limacodidae in North America typically reside on the east coast, which is
known for its dense vegetation. Feeding on several different species of foliage, for example white oak, red oak, and American beech to name a few, these caterpillars are known as polyphagic eaters. (Lill, 2006)This is an evolutionary advantage that
allows them to feed on nearly any plant they encounter.Species occurring outside of the United States show similar feeding patterns and feed on several native species of foliage.(Lill, 2006) In tropical environments the caterpillars are most often found feeding on eucalyptus and gum trees, which provide all the essential nutrients needed to develop from birth to the adult form.Several species feed on poisonous plants, which they then use to produce their own toxic venom. This ingenious ability to produce powerful poison is not unique to the order Lepidoptera, but is also a characteristic of several other species of insects. A study conducted in June of 2006 revealed that Limacodidae show preference to different leaf textures. (Lill, 2006) The team of scientists tested the effects of leaf pubescence by sampling larvae in the Ozark Mountains of Missouri for more than ten years. One specific experiment demonstrated that the caterpillars responded more positively to leafs without the fine protruding hairs. (Lill, 2006) The team was able to observe their findings in nature and noted that they often found dense populations feeding on plants with smooth leaves.As they mature they become solitary feeders and are often found feeding alone. This is an advantage because they rarely compete for resources.
As the larvae begin to develop they require more nutrition for obvious
reasons. The maturing caterpillars consume the most before preparing for their final transformation into their adult forms.During this time plants undergo extreme
feeding pressures, which often devastates the vegetation. (Rieger, 2006) Farmers consider Limacodidae as pest and often seek methods for removal such as pesticides. (Heppner, 1995) Geography:
These caterpillars prefer tropical climates and most species have been
identified throughout these areas. In tropical climates temperatures are relatively constant, thus producing a dependable and consistent environment for Limacodidae to thrive. An abundance of plants cover the floor and provide optimal protection from various predators. As herbivores they rarely ever face food shortages and feed on numerous native plants.
Not limited to tropical climates these caterpillars also occur throughout the
United StatesThere are currently 50 known species in North America, 6 of which contain urticating stinging spines. The most common Limacodidae encountered in the U.S. is the saddleback caterpillar, which has been known to surprise gardeners by clinging to the undersides of leaves. The state bearing the largest populations of Limacodidae is Florida, which is home to nearly 30 different species. (Heppner, 1995) Mating:
Mating does not occur until the immature caterpillar transforms into the
adult moth. At this stage the reproductive organs are fully developed and the adult
moth works quickly to find a mate.This can be a difficult task due to high predation and short lifespan.
In 1996 a team of three entomologists studied the complete lifecycle
ofSibinemegasomoides from egg to adult. A native of Costa Rica, Sibinemegasomoides most closely resembles the North American species Sibinestimulea, and is also equipped with stinging spines.They recorded weights at each stage of development until it was a mature adult. Once the adult moth emerged from its cocoon it dried its wings and went in search of a mate. As soon as the females mated they were busy laying eggs the next day, and this behavior lasted up to three days.(Mexzón, 1996) The scientists noted that the females of this particular species were capable of laying 7 to 15 eggs at a time and laid roughly 20 clusters of eggs on the first day. The number of clusters decreased each day the female was actively laying eggs.(Mexzón, 1996) Each egg was covered in yellow mucus, which acts as an adhesive to bind the egg to the leaf. Once successfully mated the moth dies within several days leaving its offspring to carry on the family genes. Parasites: One of the most obscure parasite interactions occurs in Lipidoptera.Several species of caterpillars often become victim to the braconid wasp (Cotesiacongregata).This small wasp is capable of injecting a virus into the circulatory system along with its young into its host with one swift jab.(Beckage,
1987) The virus that is injected causes a great deal of complications in the caterpillar and blocks any internal defensive mechanisms they have evolved. Along with the virus the wasp also injects fertilized eggs into the host.These eggs use the protein rich caterpillar as a food source, and consume their host from the inside out.As the eggs begin to develop inside the caterpillars’ body they produce large cells called teratocytes, which become visible on the body of the host as the eggs mature.(Beckage, 1987) Once the eggs develop into their larval form they begin to emerge from the inside of the caterpillar through the large cellular membranes located on the dorsal side. This brutal form of parasitism is unique toCotesiacongregata and is always fatal to the caterpillar. Defense Mechanisms:
Planet Earth has been around for nearly 4.6 billion years and throughout this
time species have been competing for resources in order to survive. Evolution has produced some of the most advanced forms of defense, which include both chemical and non‐chemical defense mechanisms. (Niehuis, 2006) This dangerous and highly competitive world has allowed the family Limacodidae to display some of the most advanced and obscure morphologies known to man.
Slug caterpillars living in dense vegetation often face numerous predators.
Birds, wasps, and other insects are just a few predators they encounter on a daily basis. Caterpillars are excellent sources of protein, which makes for a nutritional
meal even some indigenous humans cannot resist. With such high predation pressures the family Limacodidae had no other choice but to evolve sophisticated weaponry. (Murphy, 2009) One of the most notable defense mechanisms in this family are the numerous stinging spines several species display. These spines, also known as urticating hairs, are hollow extensions of their body. The fragile spines are not present at birth, but develop and become more pronounced after each consecutive molt. (Murphy, 2009) Each hair often contains a poisonous toxin that can produce an irritating sensation much like a bee sting. These fragile hairs have evolved to break off into whatever they come in contact with leaving the predator in extreme discomfort. (Heppner, 1995) In some species the venomous sac remains in contact once the spine has penetrated its intruder and continues to pulse the poison into the body. As mentioned earlier, only 6 species of stinging Limacodidae live in North America, but many others occur throughout the world. Ranging in a vast spectrum of color these caterpillars flaunt some of the most brilliant color schemes. This however is not for our viewing pleasure, but a survival strategy. Their magnificent coloration is key to their survival and allows them to blend in with various species of vegetation. Others display bright vibrant colors to warn predators of their distastefulness. (Wiklund and Järvi, 1982) This strategy is know as aposematic coloration and is used throughout the animal kingdom.Cyptic coloration is another type of camouflage used in many species of Lipidoptera. It allows them to remain unseen to the untrained predator by blending in with its
surroundings. These types of coloration are also seen in the adult moth or butterfly forms as well. They also display various body morphologies such as a retractable head. The head of Limacodidae tucks into the anterior part of the thorax when threatened. (Murphy, 2009) This morphology protects the most vulnerable and precious part of their body. Another adaptation is the ability to appear larger. The saddleback caterpillar is a perfect example, which has a large redish‐brown “saddle‐like” spot on its dorsal surface. This confuses the potential predator and is often a very successful defense mechanism. Transformations:
The caterpillar is one of the several insects that undergo a complete
metamorphosis throughout their lifetime. The changes are drastic at each stage of the process, which starts from the egg. (Nijhout, 1981) The eggs are laid by the mother on a preferred host plant that will provide the critical nutrients needed for the remainder of the transformation to adulthood.
Once the larvae emerge from the egg they almost immediately start to eat.
This stage is known as the first instar. The mass accumulation of nutrients initiates rapid growth and allows the caterpillar to transform into its next critical stage of life. The larvae eventually outgrow their outermost skin and begin to molt by shedding the outgrown skin, which defines the second instar. (Nijhout, 1981) This happens several more times throughout their lifetime. Each molt produces unique characteristics such as coloration.The number of instars depends on the species, but
5 to 7 instars are average in Lepidoptera. The last molt is known as the chrysalis. It is at this point where the newly formed skin hardens to produce the most phenomenal transformation into the adult moth or butterfly. Limacodidae are referred to as cup moths because their chrysalises are round and cup‐shaped. After a few critical days the adult has emerged and is ready for its first flight. It then focuses all its energy into finding a mate before it is eaten by a predator or out lives its short lifespan. Conclusion:
Over several millions of years Limacodidae have been evolving into some of
the most spectacular insects in the insect world. Their striking colors and morphologies are truly a work of art, but to the caterpillar these are survival strategies. They have also adapted elaborate defense mechanisms to cope with various predators such as stinging spines. All these adaptations are more than likely results of evolutionary pressures, which has created the many variations we discussed.
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