Editing Red imported fire ant (section)

==Description==
[[File:Solenopsis invicta - fire ant worker.jpg|thumbnail|left|Closeup face view of a worker]]

Red imported fire ant workers range in size from small to medium, making them [[Polymorphism (biology)|polymorphic]]. Workers measure between {{convert|2.4|and|6.0|mm|in|abbr=on}}.<ref>{{cite book|last1=Hedges|first1=S.A.|editor1-last=Moreland|editor1-first=D. |title= Handbook of Pest Control |date=1997 |publisher= Mallis Handbook and Technical Training Company |pages= 531–535 |edition= 8th}}</ref> The head measures {{cvt|0.66|to|1.41|mm|in|abbr=on}} and is {{cvt|0.65|to|1.43|mm|in|abbr=on}} wide. In the larger workers (as in the major workers), their heads measure {{cvt|1.35|to|1.40|mm|in|abbr=on}} and {{cvt|1.39|to|1.42|mm|in|abbr=on}} wide. The antenna [[Antenna (biology)#Structure|scape]]s measure {{cvt|0.96|to|1.02|mm|in|abbr=on}} and the [[Thorax|thoracic]] length is {{convert|1.70|to|1.73|mm|in|abbr=on}}.<ref name="Buren_1972">{{cite journal|last1=Buren|first1=W.F.|title=Revisionary studies on the taxonomy of the imported fire ants|journal=Journal of the Georgia Entomological Society|date=1972|volume=7|pages=1–26|doi=10.5281/zenodo.27055}}</ref> The head becomes wider behind the eyes with rounded [[occipital lobe]]s present, and unlike the similar-looking ''S. richteri'', the lobes peak further than the midline, but the occipital excision is not as crease-like. The scapes in major workers do not extend beyond occipital peak by one or two scape diameters; this feature is more noticeable in ''S. richteri''. In medium-sized workers, the scapes reach the occipital peaks and exceed the rear border in the smallest workers. In small and medium workers, the head tends to have more elliptical sides. The head of small workers is wider out front than it is behind.<ref name="Buren_1972"/> In the major workers, the [[pronotum]] does not have any angular shoulders, nor does it have any sunken posteromedian area. The promesonotum is convex and the [[propodeum]] base is rounded and also convex. The base and declivity are of equal length. The [[Suture (anatomy)|suture]] of the promesonotum is either strong or weak in larger workers. The [[Petiole (insect anatomy)|petiole]] has a thick and blunt scale; if observed from behind, it is not as rounded above in contrast to ''S. richteri'', and sometimes it may be subtruncate. The postpetiole is large and broad, and in the larger workers, it is broader than its length. The postpetiole tends to be less broad in front and broader behind. On the rear side of the dorsal surface, a transverse impression is present. In ''S. richteri'', this feature is also present but much weaker.<ref name="Buren_1972"/>

The sculpture is very similar to ''S. richteri''.<ref name="Buren_1972"/> The punctures are from where pilosity arises, and these are often elongated on the dorsal and ventral portions of the head. On the thorax, striae are present, but they are less engraved with fewer punctures than in ''S. richteri''. On the petiole, the punctates are located on the sides. The postpetiole, when viewed above, has a strong shagreen with distinct transverse punctostriae. The sides are covered in deep punctures, where they appear smaller but deeper. In ''S. richteri'', the punctures are larger and more shallow. This gives a more opaque appearance to the surface. In some cases, punctostriae may be present around the rear portion.<ref name="Buren_1972"/> The pilosity appears similar to that of ''S. richteri''. These hairs are erect and vary in length, appearing long on each side of the [[Prothorax|pronotum]] and [[mesonotum]]; on the head, the long hairs are seen in longitudinal rows. Numerous appressed pubescent hairs are on the petiolar scale; this is the opposite in ''S. richteri'', as these hairs are sparse.<ref name="Buren_1972"/> Workers appear red and somewhat yellowish with a brown or completely black [[Gaster (insect anatomy)|gaster]].<ref name="Santschi_1916">{{cite journal|last1=Santschi|first1=F.|title=Formicides sudaméricains nouveaux ou peu connus|journal=Physis|date=1916|volume=2|pages=365–399|doi=10.5281/ZENODO.14374|url=https://archive.org/details/ants_03656}}</ref> Gastric spots are sometimes seen in larger workers, where they are not as brightly coloured as those in ''S. richteri''. The gastric spot usually covers a small portion of the first gastric [[Tergum|tergite]]. The thorax is concolorous, ranging from light reddish-brown to dark-brown. The legs and [[Arthropod leg#Coxa|coxae]] are usually lightly shaded. The head has a consistent colour pattern in large workers, with the [[Insect morphology#Head|occiput]] and [[Insect morphology#Head|vertex]] appearing brown. Other parts of the head, including the front, genae, and the central region of the [[Clypeus (arthropod anatomy)|clypeus]], are either yellowish or yellowish brown. The anterior borders of the genae and mandibles are dark-brown; they also both appear to share the same coloured shade with the occiput. The scapes and [[Antenna (biology)#Structure|funiculi]] range from being the same colour as the head or shares the same shade with the occiput. Light-coloured areas of the head in small to medium-sized workers is restricted to only the frontal region, with a dark mark resembling an arrow or rocket being present. On occasion, nests may have a series of different colours. For example, workers may be much darker, and the gastric spot may be completely absent or appear dark-brown.<ref name="Buren_1972"/>

Queens have a head length of {{convert|1.27|to|1.29|mm|in|abbr=on}} and a width of {{cvt|1.32|to|1.33|mm|in|abbr=on}}.<ref name="Buren_1972"/> The scapes measure {{cvt|0.95|to|0.98|mm|in|abbr=on}} and the thorax is {{cvt|2.60|to|2.63|mm|in|abbr=on}}. The head is almost indistinguishable from ''S. richteri'', but the occipital excision is less crease-like and the scapes are considerably shorter. Its petiolar scale is convex and resembles that of ''S. richteri''. The postpetiole has straight sides that never concave, unlike in ''S. richteri'' where they concave. The thorax is almost identical, but the clear space between the metapleural striate area and propodeal [[Spiracle (arthropods)|spiracle]]s is either a narrow crease or not present. The side portions of the petiole are punctate. The sides of the postpetiole are [[Opacity (optics)|opaque]] with punctures present, but no irregular roughening is seen. The anterior of the dorsum is [[shagreen]], and the middle and rear regions bear transverse puncto-striae. All these regions have erect hairs. The anterior portions of both the petiole and postpetiole have appressed pubescence that is also seen on the propodeum. The colour of the queen is similar to that of a worker: the gaster is dark brown and the legs, scapes, and thorax are light brown with dark streaks on the [[mesothorax|mesoscutum]]. The head is yellowish or yellowish-brown around the central regions, the occiput and mandibles are a similar colour to the thorax, and the wing veins range from colourless to pale brown.<ref name="Buren_1972"/> Males appear similar to ''S. richteri'', but the upper borders of the petiolar scales are more concave. In both species, the postpetiole's and petiole's spiracles strongly project. The whole body of the male is concolorous black, but the antennae are whitish. Like the queen, the wing veins are colourless or pale brown.<ref name="Buren_1972"/>

[[File:Solenopsis invicta casent0005804 profile 1.jpg|thumbnail|Whole body image of a worker]]

The red imported ant can be misidentified as the similar-looking ''S. richteri''.<ref name="Buren_1972"/> The two species can be distinguished from each other through morphological examinations of the head, thorax, and postpetiole. In ''S. richteri'', the sides of the head are broadly elliptical and the cordate shape seen in the red imported fire ant is absent. The region of the occipital lobes that are situated nearby the midline and occipital excision appear more crease-like in ''S. richteri'' than it does in the red imported fire ant. The scapes of ''S. richteri'' are longer than they are in the red imported fire ant, and the pronotum has strong angulate shoulders. Such character is almost absent in the red imported fire ant. A shallow but sunken area is only known in the larger workers of ''S. richteri'', which is located in the posterior region of the [[Anatomical terms of location#Dorsal and ventral|dorsum]] of the pronotum. This feature is completely absent in larger red imported fire ant workers. The red imported fire ant's promesonotum is strongly convex, whereas this feature is weakly convex in ''S. richteri''. Upon examination, the base of the propodeum is elongated and straight in ''S. richteri'', while convex and shorter in the red imported fire ant. It also has a wide postpetiole with either straight or diverging sides. The postpetiole in ''S. richteri'' is narrower with converging sides. In ''S. richteri'', the transverse impression on the posterodorsal portion of the postpetiole is strong, but weak or absent in the red imported fire ant.<ref name="Buren_1972"/> As well as that, ''S. richteri'' workers are 15% larger than red imported fire ant workers, are blackish-brown, and have a yellow stripe on the dorsal side of the gaster.{{sfn|Tschinkel|2006|p=24}}{{sfn|Buhs|2005|p=11}}

===Brood===
[[File:Invicta hatching.tif|thumb|Electron micrograph of a hatching fire ant egg]]

Eggs are tiny and oval-shaped, remaining the same size for around a week. After one week, the egg assumes the shape of an embryo and forms as a larva when the egg shell is removed.<ref name="Petralia_Vinson_1979">{{cite journal|last1=Petralia|first1=R. S.|last2=Vinson|first2=S. B.|title=Developmental morphology of larvae and eggs of the Imported fire ant, ''Solenopsis invicta''|journal=Annals of the Entomological Society of America|date=1979|volume=72|issue=4|pages=472–484 |doi= 10.1093/aesa/72.4.472 |url= https://archive.org/details/ants_10306}}</ref> Larvae measure {{cvt|3|mm|in|abbr=on}}.<ref name="Wheeler_Wheeler_1977">{{cite journal |last1= Wheeler |first1=G.C. |last2=Wheeler |first2=J. |title= Supplementary studies on ant larvae: Myrmicinae |journal=Transactions of the American Entomological Society |date=1977 |volume=103 |pages= 581–602 |doi= 10.5281/zenodo.25100}}</ref> They show a similar appearance to ''[[Solenopsis geminata|S. geminata]]'' larvae, but they can be distinguished by the integument with [[spinule]]s on top of the dorsal portion of the posterior somites. The body hairs measure {{cvt|0.063|to|0.113|mm|in|abbr=on}} with a denticulate tip. The antennae both have two or three [[sensilla]]. The [[Labrum (arthropod mouthpart)|labrum]] is smaller with two hairs on the anterior surface that are {{cvt|0.013|mm|in}}. The [[Maxilla (arthropod mouthpart)|maxilla]] has a sclerotised band between the cardo and stipes. The [[wikt:labium|labium]] also has a small sclerotised band.<ref name="Wheeler_Wheeler_1977"/> The tubes of the [[labial gland]]s are known to produce or secrete a proteinaceous substance that has a rich level of digestive enzymes, which includes proteases and amylases that function as an extraintestinal digestion of solid food. The midgut also contains [[amylase]]s, roteases and upases. The narrow cells in its reservoir have little to no function in secretion.<ref name="Petralia_et_al_1980">{{cite journal |last1=Petralia |first1=R.S. |last2=Sorensen |first2=A.A. |last3=Vinson |first3=S.B. |title= The labial gland system of larvae of the imported fire ant, ''Solenopsis invicta'' Buren |url=https://archive.org/details/sim_cell-and-tissue-research_1980_206_1/page/145 |journal= Cell and Tissue Research |date=1980 |volume=206 |issue=1 |pages=145–156 |doi= 10.1007/BF00233615 |pmid= 6153574|s2cid=28786982 }}</ref> The pupae resemble adults of any caste, except that their legs and antennae are held tightly against the body. They appear white, but over time, the pupa turns darker when they are almost ready to mature.<ref name="Greenberg_Kabashima_2014">{{cite web |last1= Greenberg |first1=L. |last2= Kabashima |first2= J.N. |title= Pest Notes: Red Imported Fire Ant |url= http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7487.html|work=Statewide Integrated Pest Management Program|publisher=Agriculture and Natural Resources, University of California |access-date= 4 April 2016 |date=2014}}</ref>

Four larval [[instar]]s have been described based on distinctive morphological characters.<ref name="Petralia_Vinson_1979"/><ref>{{cite journal|last1=O'Neal|first1=J.|last2=Markin|first2=G.P. |title= The larval instars of the imported fire ant, ''Solenopsis invicta'' Buren (Hymenoptera: Formicidae) |journal= Journal of the Kansas Entomological Society |date=1975 |volume=48 |issue=2 |pages=141–151 |jstor= 25082731}}</ref><ref name="Foxetal2012">{{cite journal |last1=Fox |first1=Eduardo G P |last2=Solis |first2=Daniel Russ |last3=Rossi |first3=Mônica Lanzoni |last4=Delabie |first4=Jacques Hubert Charles |last5=de Souza |first5=Rodrigo Fernando |last6=Bueno |first6=Odair Correa |title=Comparative Immature Morphology of Brazilian Fire Ants (Hymenoptera: Formicidae: Solenopsis) |journal=Psyche: A Journal of Entomology |date=2012 |volume=2012 |pages=1–10 |doi=10.1155/2012/183284|doi-access=free |hdl=11449/73193 |hdl-access=free }}</ref> The larvae of the minor and major workers are impossible to distinguish before the final instar, when size differences become apparent.<ref name="Foxetal2012" /> Upon pupation a wider head width difference between castes become more evident. Reproductive larvae are larger than worker larvae, and present discrete morphological differences in mouthparts.<ref name="Foxetal2012" /> Fourth-instar larvae of males and queens can be differentiated based on their relative shape and body coloration,<ref name="Foxetal2012" /> and also internal gonopodal imaginal discs can differ.

===Polymorphism===
[[File:Anatomical differences of S. invicta workers.png|thumbnail|230px|left|Anatomical differences of red imported fire ant workers: The scale bar is 1 mm.]]

The red imported fire ant is polymorphic with two different castes of workers: minor workers and major workers (soldiers). Like many ants that exhibit polymorphism, young, smaller ants do not forage and tend to the brood instead, while the larger workers go out and forage.<ref>{{cite journal|last1=Mirenda|first1=J.T.|last2=Vinson|first2=S.B.|title=Division of labour and specification of castes in the red imported fire ant ''Solenopsis invicta'' Buren|journal=Animal Behaviour|date=1981|volume=29|issue=2|pages=410–420|doi=10.1016/S0003-3472(81)80100-5|s2cid=53199246}}</ref>{{sfn|Schmid-Hempel|1998|p=144}}{{sfn|Hölldobler|Wilson|1990|p=311}}{{sfn|Hölldobler|Wilson|1990|p=318}} In incipient colonies, polymorphism does not exist, but instead they are occupied by monomorphic workers called "minims" or "nanitics". The average head-width in tested colonies increases during the first six months of development.<ref name="Tschinkel_1988">{{cite journal|last1=Tschinkel|first1=W.R.|title=Colony growth and the ontogeny of worker polymorphism in the fire ant, ''Solenopsis invicta''|url=https://archive.org/details/sim_behavioral-ecology-and-sociobiology_1988-02_22_2/page/103|journal=Behavioral Ecology and Sociobiology|date=1988|volume=22|issue=2|pages=103–115|doi=10.1007/BF00303545|bibcode=1988BEcoS..22..103T |s2cid=8455126}}</ref><ref name="Tschinkel_et_al_2003"/> In five-year-old colonies, the head width of minor workers decreases, but for major workers, the head-width remains the same. The total weight of a major worker is twice that of a minor worker when they first arrive, and by six months, major workers are four times heavier than minor workers. Once major workers develop, they can make up a large portion of the workforce, with as many as 35% being major workers in a single colony.<ref name="Tschinkel_1988"/> This does not affect colony performance, as polymorphic colonies and nests with small workers produce broods at roughly the same rate, and polymorphism is not an advantage or disadvantage when food sources are not limited. However, polymorphic colonies are more energetically efficient, and under conditions where food is limited, polymorphism may provide a small advantage in brood production, but this depends on the levels of food stress.<ref>{{cite journal|last1=Porter|first1=S.D.|last2=Tschinkel|first2=W.R.|title=Fire ant polymorphism: the ergonomics of brood production|url=https://archive.org/details/sim_behavioral-ecology-and-sociobiology_1985-04_16_4/page/323|journal=Behavioral Ecology and Sociobiology|date=1985|volume=16|issue=4|pages=323–336|doi=10.1007/BF00295545|jstor=4599785|bibcode=1985BEcoS..16..323P |s2cid=5967997}}</ref>

As worker ants grow to larger sizes, the shape of the head changes, due to the head length growing at the same time as the total body length, and the head width may grow by 20%. The length of the antennae only grows slowly; the antennae may only grow 60% longer by the time the body doubles its length, thus the relative antennal length decreases by 20% as the length of the body doubles.<ref name="Tschinkel_et_al_2003">{{cite journal|last1=Tschinkel|first1=W.R.|last2=Mikheyev|first2=A.S.|last3=Storz|first3=S.R.|title=Allometry of workers of the fire ant, ''Solenopsis invicta''|journal=Journal of Insect Science|date=2003|volume=3|issue=2|pages=2|doi=10.1673/031.003.0201|pmid=15841219|pmc=524642}}</ref> All individual legs of the body are isometric with body length meaning that even when the length of the body doubles, the legs will also double. However, not all of the legs are the same length; the prothoracic portion accounts for 29% of leg length, the mesothoracic 31%, and the metathoracic 41%. The first two pairs of legs are of equal length to one another, whereas the final pair is longer.<ref name="Tschinkel_et_al_2003"/> Overall, the morphological appearance of a worker changes dramatically when it grows larger. The head exhibits the greatest shape change and the height of the [[Notum|alinotum]] grows quicker than its length, where a height/length ratio of 0.27 in minor workers and 0.32 in major workers is seen.<ref name="Tschinkel_et_al_2003"/> Due to this, larger workers tend to have a humped-shape and robust alinotum in contrast to smaller workers. No petiole segment exhibits any change in shape as the size of the body changes. The width of the gaster grows more rapidly than its length, where the width may be 96% of its length but increases to 106%.<ref name="Tschinkel_et_al_2003"/>

===Physiology===
[[File:Photomicrograph of sting apparatus components journal.pone.0050400.png|thumb|200px|Photomicrograph of sting apparatus components]]

Like other insects, the red imported fire ant breathes through a system of gas-filled tubes called [[Invertebrate trachea|tracheae]] connected to the external environment through spiracles. The terminal tracheal branches ([[tracheole]]s) make direct contact with internal organs and tissue. The transport of oxygen to cells (and carbon dioxide out of cells) occurs through [[diffusion]] of gases between the tracheoles and the surrounding tissue and is assisted by a [[discontinuous gas exchange]].<ref name="Vogt_Appel_2000">{{cite journal|last1=Vogt|first1=J.T.|last2=Appel|first2=A.G.|title=Discontinuous gas exchange in the fire ant, ''Solenopsis invicta'' Buren: Caste differences and temperature effects|url=https://archive.org/details/sim_journal-of-insect-physiology_2000-04_46_4/page/403|journal=Journal of Insect Physiology|date=2000|volume=46|issue=4|pages=403–416|doi=10.1016/S0022-1910(99)00123-7|pmid=12770204|bibcode=2000JInsP..46..403V }}</ref> As with other insects, the direct communication between the tracheal system and tissues eliminates the need for a circulating fluid network to transport O<sub>2</sub>.<ref name="Klowden_2007">{{cite book|last1=Klowden|first1=M.J.|title=Physiological Systems in Insects|url=https://archive.org/details/physiologicalsys00mjkl|date=2007|publisher=Elsevier/Academic Press|location=Amsterdam; Boston|isbn=978-0-12-369493-5|pages=[https://archive.org/details/physiologicalsys00mjkl/page/n367 357]–383, 433–449}}</ref> Thus, red imported fire ants and other arthropods can have a modest circulatory system though they have highly expensive metabolic demands.<ref>{{cite book|last1=Hill|first1=R.W.|last2=Wyse|first2=G.A.|last3=Anderson|first3=M.|title=Animal Physiology|url=https://archive.org/details/animalphysiology0000hill|date=2012|publisher=Sinauer Associates|location=Sunderland, Massachusetts|isbn=978-0-87893-662-5|pages=[https://archive.org/details/animalphysiology0000hill/page/612 612]–614|edition=3rd}}</ref>

The [[excretory system]] consists of three regions. The basal region has three cells found within the posterior portion of the midgut. The anterior and superior cavities are formed by the bases of four [[Malpighian tubule system|Malpighian tubule]]s.<ref name="Vinson_1983">{{cite journal|last1=Vinson|first1=S.B.|title=The physiology of the imported fire ant revisited|journal=The Florida Entomologist|date=1983|volume=66|issue=1|pages=126–139|doi=10.2307/3494559|jstor=3494559}}</ref> The superior cavity opens into the [[Lumen (anatomy)|lumen]] of the small intestine. The rectum is a large but thin-walled sac that occupies the posterior fifth of the larvae. The release of waste is controlled by the [[Transverse folds of rectum|rectal valves]] that lead to the anus.<ref name="Vinson_1983"/> Sometimes, the larvae secrete a liquid that consists of [[uric acid]], water and salts.<ref>{{cite journal|last1=Petralia|first1=R.S.|last2=Williams|first2=H.J.|last3=Vinson|first3=S.B.|title=The hindgut ultrastructure, and excretory products of larvae of the imported fire ant, ''Solenopsis invicta'' Buren|journal=Insectes Sociaux|date=1982|volume=29|issue=2|pages=332–345|doi=10.1007/BF02228760|s2cid=7324976}}</ref> These contents are often carried outside by workers and ejected, but colonies under water stress may consume the contents.<ref name="Vinson_1983"/> In the [[reproductive system]], queens release a [[pheromone]] that prevents dealation and [[oogenesis]] in virgin females; those tested in colonies without a queen begin [[oocyte]] development after dealation and take up the egg-laying role.<ref>{{cite journal|last1=Fletcher|first1=D.J.C.|last2=Blum|first2=M.S.|title=Pheromonal control of dealation and oogenesis in virgin queen fire ants|journal=Science|date=1981|volume=212|issue=4490|pages=73–75|doi=10.1126/science.212.4490.73|pmid=17747633|bibcode=1981Sci...212...73F}}</ref> Flight muscle degeneration is initiated by mating and juvenile hormones, and prevented by [[wikt:corpus allatum|corpus allatectomy]].<ref>{{cite journal|last1=Barker|first1=J.F.|title=Neuroendocrine regulation of oocyte maturation in the imported fire ant ''Solenopsis invicta''|journal=General and Comparative Endocrinology|date=1978|volume=35|issue=3|pages=234–237|doi=10.1016/0016-6480(78)90067-9|pmid=689357}}</ref><ref>{{cite journal|last1=Jones|first1=R.G.|last2=Davis|first2=W.L.|last3=Hung|first3=A.C.F.|last4=Vinson|first4=S.B.|title=Insemination-induced histolysis of the flight musculature in fire ants (''Solenopsis'', spp.): An ultrastructural study|journal=American Journal of Anatomy|date=1978|volume=151|issue=4|pages=603–610|doi=10.1002/aja.1001510411|pmid=645619}}</ref> [[Histolysis]] begins with the dissolution of the [[myofibril]] and the slow breakdown of the [[myofilament]]s. Such dissolution continues until it reaches the only free Z-line materials, which would also disappear; only the [[Nucleus (neuroanatomy)|nuclei]] and [[lamellar bodies]] remain.<ref name="Vinson_1983"/> In one study, the [[amino acid]]s increase in the [[hemolymph]] after insemination.<ref>{{cite journal|last1=Toom|first1=P.M.|last2=Johnson|first2=C.P.|last3=Cupp|first3=E.W.|title=Utilization of body reserves during preoviposition activity by ''Solenopsis invicta''|journal=Annals of the Entomological Society of America|date=1976|volume=69|issue=1|pages=145–148|doi=10.1093/aesa/69.1.145}}</ref>

The [[Endocrine system|glandular system]] contains four glands: the mandibular, maxillary, labial, and [[Pharynx|postpharyngeal]] glands.<ref name="Vinson_1983"/> The postpharyngeal is well developed in the queen, while the other glands are larger in workers. The postpharyngeal gland functions as a vacuum to absorb [[fatty acid]]s and [[triglyceride]]s, as well as a gastric [[Cecum|caecum]].<ref name="Vinson_1980">{{cite journal|last1=Vinson|first1=S.B.|last2=Phillips|first2=S.A.|last3=Williams|first3=H.J.|title=The function of the post-pharyngeal glands of the red imported fire ant, ''Solenopsis invicta'' buren|journal=Journal of Insect Physiology|date=1980|volume=26|issue=9|pages=645–650|doi=10.1016/0022-1910(80)90035-9|bibcode=1980JInsP..26..645V }}</ref> The functions of the other glands remain poorly understood. In one study discussing the enzymes of the digestion system of adult ants, [[lipase]] activity was found in the mandibular and labial glands, as well as [[invertase]] activity. The [[Dufour's gland]] found in the ant acts as a source of [[trail pheromone]]s, although scientists believed the poison gland was the source of the queen pheromone.<ref name="Vinson_1983"/><ref>{{cite journal|last1=Vander Meer|first1=R.K.|last2=Lofgren|first2=C.S.|title=Biochemical and behavioral evidence foe hybridization between fire ants, ''Solenopsis invicta'' and ''Solenopsis richteri'' (Hymenoptera: Formicidae)|journal=Journal of Chemical Ecology|date=1989|volume=15|issue=6|pages=1757–1765|doi=10.1007/BF01012263|pmid=24272179|bibcode=1989JCEco..15.1757V |s2cid=23144401}}</ref><ref name="Vander">{{cite journal|last1=Vander Meer|first1=R.K.|last2=Glancey|first2=B.M.|last3=Lofgren|first3=C.S.|last4=Glover|first4=A.|last5=Tumlinson|first5=J.H.|last6=Rocca|first6=J.|title=The poison sac of red imported fire ant queens: source of a pheromone attractant|journal=Annals of the Entomological Society of America|date=1980|volume=73|issue=5|pages=609–612|doi=10.1093/aesa/73.5.609}}</ref> The neurohormone [[pheromone biosynthesis activating neuropeptide]] is found in the ant that activates the biosynthesis of pheromones from the Dufour's gland.<ref>{{cite journal|last1=Choi|first1=M.Y.|last2=Vander Meer|first2=R.K.|last3=Renou|first3=M.|title=Ant trail pheromone biosynthesis is triggered by a neuropeptide hormone|journal=PLOS ONE|date=2012|volume=7|issue=11|pages=e50400|doi=10.1371/journal.pone.0050400|pmid=23226278|pmc=3511524|bibcode=2012PLoSO...750400C|doi-access=free}}</ref> The [[spermatheca]] gland is found in queens, which functions in sperm maintenance. Males appear to lack these glands, but those associated with its head are morphologically similar to those found in workers, but these glands may act differently.<ref name="Vinson_1980"/>

[[File:A Comparative Study between Solenopsis invicta and Solenopsis richteri on Tolerance to Heat and Desiccation Stresses journal.pone.0096842.png|thumb|left|200px|Water loss rates of workers and female [[alate]]s in ''S. invicta'' and ''S. richteri'']]

The ant faces many respiratory challenges due to its highly variable environment, which can cause increased [[desiccation]], [[Hypoxia (medical)|hypoxia]], and [[hypercapnia]]. Hot, humid climates cause an increase in heart rate and respiration which increases energy and water loss.<ref name="Klowden_2007"/><ref>{{cite journal|last1=Elzen|first1=G.W|title=Oxygen consumption and water loss in the imported fire ant ''Solenopsis invicta'' Buren|journal=Comparative Biochemistry and Physiology A|date=1986|volume=84|issue=1|pages=13–17|doi=10.1016/0300-9629(86)90035-6}}</ref> Hypoxia and hypercapnia can result from red imported fire ant colonies living in poorly ventilated [[Thermoregulation|thermoregulatory]] mounds and underground nests. Discontinuous gas exchange (DGE) may allow ants to survive the hypercapnic and hypoxic conditions frequently found in their burrows;<ref name="Vogt_Appel_2000"/> it is ideal for adapting to these conditions because it allows the ants to increase the period of O<sub>2</sub> intake and CO<sub>2</sub> expulsion independently through spiracle manipulation. The invasion success of the red imported fire ant may possibly be related to its physiological tolerance to [[abiotic stress]], being more heat tolerant and more adaptable to desiccation stress than ''S. richteri''. This means that the ant is less vulnerable to heat and desiccation stress. Although ''S. richteri'' has higher water body content than the red imported fire ant, ''S. richteri'' was more vulnerable to desiccation stress. The lower sensitivity to desiccation is due to a lower water loss rate.<ref>{{cite journal|last1=Chen|first1=J.|last2=Rashid|first2=T.|last3=Feng|first3=G.|last4=Hughes|first4=W.|title=A comparative study between ''Solenopsis invicta'' and ''Solenopsis richteri'' on tolerance to heat and desiccation stresses|journal=PLOS ONE|date=2014|volume=9|issue=6|pages=e96842|doi=10.1371/journal.pone.0096842|pmid=24915009|pmc=4051589|bibcode=2014PLoSO...996842C|doi-access=free}}</ref> Colonies living in unshaded and warmer sites tend to have a higher heat tolerance than those living in shaded and cooler sites.<ref>{{cite journal|last1=Boyles|first1=J.G.|last2=Aubrey|first2=D.P.|last3=Hickman|first3=C.R.|last4=Murray|first4=K.L.|last5=Timpone|first5=J.C.|last6=Ops|first6=C.H.|title=Variation in physiological response of red imported fire ants (''Solenopsis invicta'') to small-scale thermal heterogeneity|journal=Journal of Thermal Biology|date=2009|volume=34|issue=2|pages=81–84|doi=10.1016/j.jtherbio.2008.10.005|bibcode=2009JTBio..34...81B }}</ref>

Metabolic rate, which indirectly affects respiration, is also influenced by environmental temperature. Peak [[metabolism]] occurs at about 32&nbsp;°C.<ref name="Poter_Tschinkel_1993">{{cite journal|last1=Porter|first1=S.D.|last2=Tschinkel|first2=W.R.|title=Fire ant thermal preferences: behavioral control of growth and metabolism|journal=Behavioral Ecology and Sociobiology|date=1993|volume=32|issue=5|page=321 |doi=10.1007/BF00183787|bibcode=1993BEcoS..32..321P |s2cid=9840395}}</ref> Metabolism, and therefore respiration rate, increases consistently as temperature increases. DGE stops above 25&nbsp;°C, although the reason for this is currently unknown.<ref name="Vogt_Appel_1999">{{cite journal|last1=Vogt|first1=J.T.|last2=Appel|first2=A.G.|title=Standard metabolic rate of the fire ant, ''Solenopsis invicta'' Buren: effects of temperature, mass, and caste|url=https://archive.org/details/sim_journal-of-insect-physiology_1999-07_45_7/page/655|journal=Journal of Insect Physiology|date=1999|volume=45|issue=7|pages=655–666|doi=10.1016/S0022-1910(99)00036-0|pmid=12770351|bibcode=1999JInsP..45..655V }}</ref>

Respiration rate also appears to be influenced significantly by caste. Males show a considerably higher rate of respiration than females and workers, due, in part, to their capability for flight and higher muscle mass. In general, males have more muscle and less fat, resulting in a higher metabolic O<sub>2</sub> demand.<ref name="Vogt_Appel_1999"/>
While the metabolic rate is highest at 32&nbsp;°C, colonies often thrive at slightly cooler temperatures (around 25&nbsp;°C). The high rate of metabolic activity associated with warmer temperatures is a limiting factor on colony growth because the need for food consumption is also increased. As a result, larger colonies tend to be found in cooler conditions because the metabolic demands required to sustain a colony are decreased.<ref name="Poter_Tschinkel_1993"/>