昆(kun)蟲種類(lei)這么多(duo)，因此(ci)，它們的(de)生(sheng)活(huo)方式與生(sheng)活(huo)場所(suo)(suo)必然是多(duo)種多(duo)樣的(de)，而且有些昆(kun)蟲的(de)生(sheng)活(huo)方式和生(sheng)活(huo)本能的(de)表現很(hen)有研究(jiu)價值，例如昆(kun)蟲易于(yu)入侵氣候環境相(xiang)似(si)的(de)地(di)域并定殖。可(ke)以說，從(cong)(cong)天(tian)涯到(dao)(dao)海(hai)角(jiao)，從(cong)(cong)高山到(dao)(dao)深(shen)淵，從(cong)(cong)赤道到(dao)(dao)兩極(ji)，從(cong)(cong)海(hai)洋、河流到(dao)(dao)沙(sha)漠，從(cong)(cong)草(cao)地(di)到(dao)(dao)森林，從(cong)(cong)野外到(dao)(dao)室內，從(cong)(cong)天(tian)空(kong)到(dao)(dao)土壤，到(dao)(dao)處都有昆(kun)蟲的(de)身(shen)影。不(bu)過，要按主要蟲態的(de)最適宜的(de)活(huo)動場所(suo)(suo)來區分，大致(zhi)可(ke)分為五類(lei)。
（1）在空中生活的昆蟲(chong)：這些昆蟲(chong)大(da)多是白天(tian)活動(dong)，成蟲(chong)期具有發達的翅膀(bang)，通常(chang)有發達的口器，成蟲(chong)壽(shou)命比較長。如蜜(mi)蜂、馬蜂、蜻蜓、蒼蠅(ying)、蚊子、牛虻(meng)、蝴(hu)蝶等。昆蟲(chong)在空中活動(dong)階段主要是進行遷移擴散，尋捕(bu)食物，婚配求(qiu)偶和選擇產卵場所。
（2）在(zai)地(di)(di)表(biao)生活(huo)的(de)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)：這類(lei)(lei)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)無翅，或有(you)(you)翅但已不善飛翔，或只能爬行(xing)和(he)(he)跳躍。有(you)(you)些善飛的(de)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)，其幼蟲(chong)(chong)(chong)(chong)(chong)期(qi)和(he)(he)蛹期(qi)也(ye)(ye)都是(shi)在(zai)地(di)(di)面(mian)生活(huo)。一些寄生性昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)和(he)(he)專以腐敗動植(zhi)物(wu)(wu)為(wei)食的(de)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)（包括與人(ren)類(lei)(lei)共同在(zai)室內(nei)生活(huo)的(de)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)），也(ye)(ye)大部分在(zai)地(di)(di)表(biao)活(huo)動。在(zai)地(di)(di)表(biao)活(huo)動的(de)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)占所有(you)(you)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)種類(lei)(lei)的(de)絕(jue)大多數，因為(wei)地(di)(di)面(mian)是(shi)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)食物(wu)(wu)的(de)所在(zai)地(di)(di)和(he)(he)棲息(xi)處。這類(lei)(lei)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)常(chang)見的(de)有(you)(you)步行(xing)蟲(chong)(chong)(chong)(chong)(chong)（放屁蟲(chong)(chong)(chong)(chong)(chong)）、蟑螂等(deng)。
（3）在(zai)土(tu)(tu)壤中生活(huo)(huo)的(de)(de)(de)昆(kun)蟲(chong)：這些昆(kun)蟲(chong)都以植(zhi)物的(de)(de)(de)根(gen)和(he)土(tu)(tu)壤中的(de)(de)(de)腐殖(zhi)質為(wei)(wei)食(shi)料。由于它(ta)(ta)們在(zai)土(tu)(tu)壤中的(de)(de)(de)活(huo)(huo)動和(he)對植(zhi)物根(gen)的(de)(de)(de)啃食(shi)而(er)成(cheng)為(wei)(wei)農業、果樹和(he)苗木的(de)(de)(de)一大(da)害。這些昆(kun)蟲(chong)最害怕(pa)光(guang)線，大(da)多數種類(lei)的(de)(de)(de)活(huo)(huo)動與遷移能力都比較差，白天(tian)很少鉆到地面活(huo)(huo)動，晚上(shang)和(he)陰雨天(tian)是它(ta)(ta)們最適宜(yi)的(de)(de)(de)活(huo)(huo)動時間。這類(lei)昆(kun)蟲(chong)常見的(de)(de)(de)有(you)螻(lou)蛄(gu)、地老虎(hu)（夜蛾(e)的(de)(de)(de)幼(you)蟲(chong)）、蟬的(de)(de)(de)幼(you)蟲(chong)等。
（4）在(zai)水中(zhong)(zhong)生(sheng)活的(de)(de)昆(kun)蟲(chong)：有的(de)(de)昆(kun)蟲(chong)終生(sheng)生(sheng)活在(zai)水中(zhong)(zhong)，如半翅目的(de)(de)負子蝽(chun)、田鱉、龜蝽(chun)、劃蝽(chun)等(deng)，鞘翅目的(de)(de)龍虱、水龜蟲(chong)等(deng)。有些昆(kun)蟲(chong)只是幼蟲(chong)（特(te)(te)稱它們(men)為稚蟲(chong)）生(sheng)活在(zai)水中(zhong)(zhong)，如蜻蜓(ting)、石蛾、蜉蝣等(deng)。水生(sheng)昆(kun)蟲(chong)的(de)(de)共同(tong)特(te)(te)點是：體側的(de)(de)氣(qi)門退化，而位(wei)于身(shen)體兩(liang)端的(de)(de)氣(qi)門發達或以特(te)(te)殊(shu)的(de)(de)氣(qi)管鰓代替(ti)氣(qi)門進行呼吸作用；大部分種類(lei)有扁平而多毛(mao)的(de)(de)游(you)泳足，起劃水的(de)(de)作用。
（5）寄(ji)(ji)生(sheng)(sheng)(sheng)性昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)：這(zhe)(zhe)類昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)的(de)(de)體(ti)型比(bi)較小(xiao)，活(huo)動(dong)能力比(bi)較差，大(da)部分種(zhong)(zhong)類的(de)(de)幼蟲(chong)(chong)(chong)(chong)(chong)都沒(mei)有(you)足(zu)或足(zu)已不再能行(xing)(xing)走，眼(yan)睛的(de)(de)視力也(ye)減弱了。有(you)些(xie)寄(ji)(ji)生(sheng)(sheng)(sheng)性昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)終生(sheng)(sheng)(sheng)寄(ji)(ji)生(sheng)(sheng)(sheng)在(zai)(zai)(zai)哺(bu)乳動(dong)物的(de)(de)體(ti)表，依靠吸(xi)血為(wei)生(sheng)(sheng)(sheng)，如跳(tiao)蚤(zao)、虱子等(deng)。有(you)的(de)(de)則寄(ji)(ji)生(sheng)(sheng)(sheng)在(zai)(zai)(zai)動(dong)物體(ti)內(nei)，如馬(ma)胃蠅(ying)。另一(yi)(yi)些(xie)昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)寄(ji)(ji)生(sheng)(sheng)(sheng)在(zai)(zai)(zai)其(qi)它昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)體(ti)內(nei)，對(dui)人(ren)類有(you)益，可(ke)利(li)用它們來防治(zhi)害蟲(chong)(chong)(chong)(chong)(chong)，稱為(wei)生(sheng)(sheng)(sheng)物防治(zhi)。這(zhe)(zhe)些(xie)昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)主要有(you)小(xiao)蜂(feng)、姬蜂(feng)、繭蜂(feng)、寄(ji)(ji)蠅(ying)等(deng)。在(zai)(zai)(zai)寄(ji)(ji)生(sheng)(sheng)(sheng)性昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)中，還(huan)有(you)一(yi)(yi)種(zhong)(zhong)叫(jiao)做重(zhong)寄(ji)(ji)生(sheng)(sheng)(sheng)的(de)(de)現(xian)象。就是當一(yi)(yi)種(zhong)(zhong)寄(ji)(ji)生(sheng)(sheng)(sheng)蜂(feng)或寄(ji)(ji)生(sheng)(sheng)(sheng)蠅(ying)寄(ji)(ji)生(sheng)(sheng)(sheng)在(zai)(zai)(zai)植食性昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)身上后，又有(you)另一(yi)(yi)種(zhong)(zhong)寄(ji)(ji)生(sheng)(sheng)(sheng)性昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)再寄(ji)(ji)生(sheng)(sheng)(sheng)于前(qian)一(yi)(yi)種(zhong)(zhong)寄(ji)(ji)生(sheng)(sheng)(sheng)昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)身上。有(you)些(xie)種(zhong)(zhong)類還(huan)可(ke)以進行(xing)(xing)二重(zhong)，或三重(zhong)寄(ji)(ji)生(sheng)(sheng)(sheng)。這(zhe)(zhe)些(xie)現(xian)象對(dui)昆(kun)(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)來說，只是為(wei)了生(sheng)(sheng)(sheng)存競爭(zheng)的(de)(de)一(yi)(yi)種(zhong)(zhong)本能。
一對觸(chu)角(jiao)，一般有兩對翅，三對足。昆蟲(chong)觸(chu)角(jiao)節數目,包括觸(chu)角(jiao)棒節數目也經常是鑒定依據。
昆(kun)蟲(chong)的(de)身體分為頭部，胸部(thorax)和腹部(也譯：下體)(abdomen)。
頭部是昆(kun)蟲身(shen)體最前(qian)面(mian)的(de)一個體段，由幾(ji)個體節(jie)組(zu)合而成，外壁堅硬，形成頭殼。頭部為(wei)感(gan)覺和取(qu)食的(de)中心，頭部的(de)上前(qian)方有(you)一對觸角(jiao)，下(xia)方是口(kou)器，具有(you)3對口(kou)器附肢和1對觸角(jiao)，兩(liang)側通常有(you)一對大(da)的(de)復眼，頭頂常常有(you)一至三個小的(de)單眼。
胸(xiong)(xiong)(xiong)部(bu)由三個(ge)(ge)體節(jie)(jie)組成，由前(qian)向后依次稱為(wei)前(qian)胸(xiong)(xiong)(xiong)(prothorax)、中(zhong)胸(xiong)(xiong)(xiong)(mesothorax)和后胸(xiong)(xiong)(xiong)(metathorax)。每個(ge)(ge)體節(jie)(jie)都帶有一(yi)對(dui)(dui)附(fu)肢，稱為(wei)胸(xiong)(xiong)(xiong)足(zu)(zu)。胸(xiong)(xiong)(xiong)足(zu)(zu)分成節(jie)(jie)段，分別(bie)為(wei)基節(jie)(jie)(coxa)、轉節(jie)(jie)(trochanter)、腿節(jie)(jie)(femur)、脛節(jie)(jie)(tibia)、跗節(jie)(jie)(tarsus)和前(qian)跗節(jie)(jie)(pretarsus)。跗節(jie)(jie)通常分為(wei)5個(ge)(ge)跗分節(jie)(jie)，有時還帶有成對(dui)(dui)的(de)爪子。胸(xiong)(xiong)(xiong)足(zu)(zu)通常會(hui)特(te)化，以更好地完成如挖，跳，游泳或(huo)是捕(bu)捉等任(ren)務。第一(yi)胸(xiong)(xiong)(xiong)節(jie)(jie)的(de)背部(bu)被稱為(wei)前(qian)胸(xiong)(xiong)(xiong)背板(pronotum)，通常會(hui)特(te)別(bie)加固。另(ling)外的(de)兩個(ge)(ge)胸(xiong)(xiong)(xiong)節(jie)(jie)的(de)背面通常會(hui)各(ge)帶有一(yi)對(dui)(dui)翅(chi)膀。在甲蟲(chong)的(de)前(qian)對(dui)(dui)翅(chi)膀之間有一(yi)塊三角形的(de)甲片。胸(xiong)(xiong)(xiong)部(bu)是運動的(de)中(zhong)心，具有3對(dui)(dui)足(zu)(zu)。
昆(kun)蟲(chong)在(zai)腹部(bu)(bu)有(you)著(zhu)重要的器(qi)官(guan)(guan)，如管(guan)狀的心臟，梯(ti)形(xing)神經(jing)系統，胃腸(chang)系統和(he)(he)生(sheng)殖器(qi)官(guan)(guan)。部(bu)(bu)分器(qi)官(guan)(guan)還會“越界”到前面去(qu)，如神經(jing)中(zhong)樞--咽上神經(jing)節或是腦部(bu)(bu)，和(he)(he)其他(ta)動物一樣位于頭部(bu)(bu)。在(zai)軀(qu)體(ti)中(zhong)還藏著(zhu)分支的氣(qi)(qi)管(guan)，它(ta)們會直接(jie)把(ba)氧氣(qi)(qi)送到身體(ti)的各個器(qi)官(guan)(guan)去(qu)。器(qi)官(guan)(guan)開口(kou)于線粒體(ti)附(fu)近，可(ke)以(yi)更快的利(li)用空氣(qi)(qi)。昆(kun)蟲(chong)在(zai)體(ti)側壁(bi)還有(you)氣(qi)(qi)孔(kong)(stigma)，直接(jie)與外(wai)界大氣(qi)(qi)接(jie)觸，可(ke)以(yi)通過(guo)肌肉收縮關(guan)閉(bi)。因為昆(kun)蟲(chong)這一套呼吸(xi)系統非(fei)常有(you)效，所以(yi)昆(kun)蟲(chong)要擔(dan)心的并(bing)不是缺(que)氧，而是過(guo)度氧化(hua)。氣(qi)(qi)孔(kong)的可(ke)關(guan)閉(bi)性(xing)使得昆(kun)蟲(chong)具有(you)暫時停止(zhi)呼吸(xi)的能力。腹部(bu)(bu)是生(sheng)殖中(zhong)心，其中(zhong)包含著(zhu)生(sheng)殖系統和(he)(he)大部(bu)(bu)分內臟，無(wu)行動用的附(fu)肢(zhi)，但多(duo)數有(you)轉化(hua)成外(wai)生(sheng)殖器(qi)的附(fu)肢(zhi)。
昆蟲體型雖小，感官卻很發(fa)達(da)。它們擁有比許(xu)多大型動物更為靈敏(min)的(de)感覺，可以看(kan)到(dao)人眼看(kan)不(bu)到(dao)的(de)光線，聽(ting)到(dao)人耳聽(ting)不(bu)到(dao)的(de)聲音，嗅(xiu)到(dao)百米(mi)之外的(de)同伴的(de)氣味。
昆(kun)蟲(chong)的(de)眼(yan)睛包括單(dan)眼(yan)和復(fu)眼(yan)，復(fu)眼(yan)由許多(duo)六角(jiao)形的(de)小眼(yan)組成，單(dan)眼(yan)有(you)背(bei)單(dan)眼(yan)和側單(dan)眼(yan)之(zhi)分。除(chu)寄生性昆(kun)蟲(chong)外(wai)，一般昆(kun)蟲(chong)都有(you)一對(dui)復(fu)眼(yan)，頭頂上還有(you)1到3個背(bei)單(dan)眼(yan)。
視覺(jue)(jue)的獨(du)特(te)(te)性，昆蟲能看見人類和絕(jue)大(da)多數動(dong)物都看不到(dao)的紫(zi)外線(xian)，而有些花瓣可以反射紫(zi)外線(xian)，昆蟲就(jiu)能依靠這(zhe)種獨(du)特(te)(te)的視覺(jue)(jue)，根據紫(zi)外線(xian)的變化找到(dao)花蜜和花粉(fen)。
有些昆蟲(chong)的(de)(de)耳(er)朵長得很奇怪，例如(ru)蟋(xi)蟀的(de)(de)耳(er)朵就是它們每條前足膝蓋以下一塊呈(cheng)鼓膜(mo)狀的(de)(de)隆起，能感覺其他蟋(xi)蟀求偶的(de)(de)聲(sheng)音(yin)(yin)；飛蛾的(de)(de)耳(er)朵長在腹(fu)部，可以感受到蝙蝠靠近的(de)(de)聲(sheng)音(yin)(yin)。
多數(shu)昆(kun)蟲(chong)在兩只復眼的中(zhong)上方都有(you)一對觸角，觸角是昆(kun)蟲(chong)的主要感(gan)覺器官，幫助昆(kun)蟲(chong)探明前方是否(fou)有(you)障礙物，尋找食物和配偶。有(you)些昆(kun)蟲(chong)也經常用觸角與(yu)同(tong)伴交流信息。
口(kou)器(qi)(qi)(qi)是昆(kun)蟲(chong)的嘴，擔(dan)負(fu)著取食(shi)的重任(ren)。因(yin)為食(shi)物不同(tong)，不同(tong)的昆(kun)蟲(chong)也就(jiu)具有不同(tong)類型的口(kou)器(qi)(qi)(qi)，如蝗蟲(chong)的咀嚼式口(kou)器(qi)(qi)(qi)，蚊、蟬的刺(ci)吸式口(kou)器(qi)(qi)(qi)，家蠅的舐吸式口(kou)器(qi)(qi)(qi)等。
溫度(du)是決定昆(kun)(kun)蟲(chong)(chong)(chong)生長發(fa)育(yu)速率(lv)的(de)(de)(de)(de)(de)最(zui)(zui)重要(yao)的(de)(de)(de)(de)(de)因(yin)子(zi)，氣(qi)(qi)(qi)(qi)候變暖能(neng)(neng)加快(kuai)昆(kun)(kun)蟲(chong)(chong)(chong)各(ge)蟲(chong)(chong)(chong)態(tai)的(de)(de)(de)(de)(de)發(fa)育(yu)，導致其首次出(chu)現期、遷飛期及種(zhong)群高峰期提前(qian)。陸生昆(kun)(kun)蟲(chong)(chong)(chong)在(zai)(zai)(zai)環(huan)境太熱(re)時尋找一個(ge)陰(yin)涼潮濕的(de)(de)(de)(de)(de)處(chu)所。如(ru)暴(bao)露在(zai)(zai)(zai)陽光(guang)(guang)下(xia)，它(ta)使(shi)自己處(chu)于(yu)(yu)體(ti)表受熱(re)面積最(zui)(zui)小的(de)(de)(de)(de)(de)位置。如(ru)太冷，昆(kun)(kun)蟲(chong)(chong)(chong)留在(zai)(zai)(zai)陽光(guang)(guang)下(xia)取暖。許多蝴蝶在(zai)(zai)(zai)飛行(xing)前(qian)需(xu)展(zhan)翅(chi)收集熱(re)量。蛾在(zai)(zai)(zai)飛行(xing)前(qian)震動翅(chi)或(huo)抖動身體(ti)，并(bing)藉毛或(huo)鱗片在(zai)(zai)(zai)身體(ti)周圍形(xing)成(cheng)一層空氣(qi)(qi)(qi)(qi)絕(jue)緣層保住體(ti)熱(re)。最(zui)(zui)適于(yu)(yu)飛行(xing)的(de)(de)(de)(de)(de)肌肉(rou)溫度(du)是38～40℃（100～104℉）。在(zai)(zai)(zai)嚴寒時，身體(ti)結凍(dong)是對昆(kun)(kun)蟲(chong)(chong)(chong)最(zui)(zui)大(da)(da)(da)的(de)(de)(de)(de)(de)危險。在(zai)(zai)(zai)寒冷地區(qu)能(neng)(neng)越冬種(zhong)類稱為(wei)耐(nai)寒昆(kun)(kun)蟲(chong)(chong)(chong)。少數昆(kun)(kun)蟲(chong)(chong)(chong)能(neng)(neng)忍受體(ti)液中(zhong)出(chu)現冰(bing)晶(jing)，不過在(zai)(zai)(zai)這種(zhong)情況下(xia)細胞內含(han)物(wu)可(ke)能(neng)(neng)并(bing)未凍(dong)結。但大(da)(da)(da)多數昆(kun)(kun)蟲(chong)(chong)(chong)的(de)(de)(de)(de)(de)耐(nai)寒意味著阻止(zhi)冰(bing)凍(dong)。抗(kang)凍(dong)作(zuo)用(yong)部(bu)分是由(you)于(yu)(yu)集聚了大(da)(da)(da)量的(de)(de)(de)(de)(de)甘油作(zuo)為(wei)抗(kang)凍(dong)劑；部(bu)分是由(you)于(yu)(yu)血液中(zhong)的(de)(de)(de)(de)(de)物(wu)理變化，溫度(du)遠在(zai)(zai)(zai)冰(bing)點(dian)之下(xia)而(er)仍不凍(dong)。防(fang)干旱包(bao)括堅硬的(de)(de)(de)(de)(de)防(fang)水(shui)(shui)蠟以及擴大(da)(da)(da)貯水(shui)(shui)的(de)(de)(de)(de)(de)機制。水(shui)(shui)生昆(kun)(kun)蟲(chong)(chong)(chong)除了步足發(fa)生顯著的(de)(de)(de)(de)(de)變化而(er)適于(yu)(yu)游泳外，主要(yao)適應性變化在(zai)(zai)(zai)于(yu)(yu)呼吸(xi)。有(you)(you)(you)的(de)(de)(de)(de)(de)升到水(shui)(shui)面呼吸(xi)。蚊只利(li)用(yong)呼吸(xi)管(guan)(guan)末端的(de)(de)(de)(de)(de)最(zui)(zui)后一對腹氣(qi)(qi)(qi)(qi)孔吸(xi)氣(qi)(qi)(qi)(qi)。龍虱(shi)在(zai)(zai)(zai)鞘翅(chi)與(yu)腹部(bu)之間(jian)有(you)(you)(you)一貯氣(qi)(qi)(qi)(qi)室。呼吸(xi)空氣(qi)(qi)(qi)(qi)的(de)(de)(de)(de)(de)昆(kun)(kun)蟲(chong)(chong)(chong)在(zai)(zai)(zai)體(ti)表的(de)(de)(de)(de)(de)毛間(jian)形(xing)成(cheng)空氣(qi)(qi)(qi)(qi)層，作(zuo)用(yong)如(ru)鰓(sai)(sai)(sai)，使(shi)它(ta)能(neng)(neng)從(cong)(cong)水(shui)(shui)中(zhong)取得氣(qi)(qi)(qi)(qi)，延長潛水(shui)(shui)的(de)(de)(de)(de)(de)時間(jian)。水(shui)(shui)中(zhong)的(de)(de)(de)(de)(de)昆(kun)(kun)蟲(chong)(chong)(chong)幼(you)(you)蟲(chong)(chong)(chong)直接從(cong)(cong)水(shui)(shui)中(zhong)得氣(qi)(qi)(qi)(qi)。搖蚊幼(you)(you)蟲(chong)(chong)(chong)整個(ge)表皮層有(you)(you)(you)豐富的(de)(de)(de)(de)(de)氣(qi)(qi)(qi)(qi)管(guan)(guan)。毛翅(chi)目（Trichoptera）和蜉蝣目（Ephemeroptera）幼(you)(you)蟲(chong)(chong)(chong)有(you)(you)(you)氣(qi)(qi)(qi)(qi)管(guan)(guan)鰓(sai)(sai)(sai)。大(da)(da)(da)型(xing)的(de)(de)(de)(de)(de)蜻蜓(ting)幼(you)(you)蟲(chong)(chong)(chong)鰓(sai)(sai)(sai)在(zai)(zai)(zai)直腸內，水(shui)(shui)從(cong)(cong)肛門進出(chu)提供氧氣(qi)(qi)(qi)(qi)。
昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)(chong)幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)的(de)(de)孵出(chu)(chu)通(tong)過不同(tong)途徑。如蛾(e)、蝶類動物(wu)咬破卵(luan)殼(ke)而(er)(er)出(chu)(chu)；蚤有(you)(you)孵化刺，用刺在(zai)(zai)(zai)(zai)殼(ke)上(shang)(shang)切一縫；有(you)(you)的(de)(de)推(tui)掉卵(luan)殼(ke)上(shang)(shang)的(de)(de)卵(luan)蓋(gai)而(er)(er)出(chu)(chu)卵(luan)。幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)孵化時(shi)能(neng)吞入空(kong)氣，以(yi)便(bian)用力掙出(chu)(chu)卵(luan)殼(ke)；在(zai)(zai)(zai)(zai)孵出(chu)(chu)后(hou)到表皮(pi)(pi)(pi)硬化前，繼續吞氣，擴張自身。表皮(pi)(pi)(pi)一旦硬化，便(bian)不能(neng)再(zai)(zai)長(chang)，只有(you)(you)通(tong)過一系列蛻(tui)(tui)皮(pi)(pi)(pi)，在(zai)(zai)(zai)(zai)蛻(tui)(tui)去舊皮(pi)(pi)(pi)，長(chang)出(chu)(chu)較大(da)的(de)(de)新皮(pi)(pi)(pi)之際才(cai)能(neng)長(chang)大(da)。蛻(tui)(tui)皮(pi)(pi)(pi)時(shi)，體(ti)形(xing)(xing)可(ke)能(neng)驟變(bian)(bian)(bian)。多(duo)數原始的(de)(de)無翅(chi)(chi)(chi)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)(chong)，如衣魚(yu)，在(zai)(zai)(zai)(zai)長(chang)大(da)為(wei)(wei)(wei)(wei)成蟲(chong)(chong)(chong)(chong)(chong)(chong)的(de)(de)過程中(zhong)身體(ti)結(jie)構幾乎沒有(you)(you)變(bian)(bian)(bian)化，稱為(wei)(wei)(wei)(wei)無變(bian)(bian)(bian)態昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)(chong)。而(er)(er)蚱蜢（直翅(chi)(chi)(chi)目(mu)Orthoptera）、蝽（異翅(chi)(chi)(chi)目(mu)Heteroptera）和蚜蟲(chong)(chong)(chong)(chong)(chong)(chong)（同(tong)翅(chi)(chi)(chi)目(mu)Homoptera），起初體(ti)形(xing)(xing)不變(bian)(bian)(bian)，直到最(zui)后(hou)才(cai)變(bian)(bian)(bian)成有(you)(you)翅(chi)(chi)(chi)的(de)(de)成蟲(chong)(chong)(chong)(chong)(chong)(chong)，生(sheng)殖器也發育(yu)成熟，稱不完全(quan)變(bian)(bian)(bian)態。高等的(de)(de)鱗(lin)翅(chi)(chi)(chi)目(mu)、鞘翅(chi)(chi)(chi)目(mu)、膜(mo)翅(chi)(chi)(chi)目(mu)和雙翅(chi)(chi)(chi)目(mu)屬(shu)于(yu)全(quan)變(bian)(bian)(bian)態，幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)完全(quan)不像成蟲(chong)(chong)(chong)(chong)(chong)(chong)；幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)經(jing)一系列蛻(tui)(tui)皮(pi)(pi)(pi)，變(bian)(bian)(bian)化較小(xiao)，然后(hou)變(bian)(bian)(bian)蛹(yong)(yong)，再(zai)(zai)變(bian)(bian)(bian)為(wei)(wei)(wei)(wei)成蟲(chong)(chong)(chong)(chong)(chong)(chong)。幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)的(de)(de)形(xing)(xing)狀多(duo)種多(duo)樣(yang)(yang)，可(ke)分(fen)為(wei)(wei)(wei)(wei)5型(xing)(xing)：蠋(zhu)型(xing)(xing)（蛾(e)、蝶類動物(wu)）、蠐螬型(xing)(xing)、衣魚(yu)型(xing)(xing)（蛃型(xing)(xing)）、叩(kou)頭蟲(chong)(chong)(chong)(chong)(chong)(chong)幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)型(xing)(xing)和蛆型(xing)(xing)。蛹(yong)(yong)分(fen)為(wei)(wei)(wei)(wei)被(bei)蛹(yong)(yong)（附(fu)(fu)肢不同(tong)程度地緊貼在(zai)(zai)(zai)(zai)體(ti)上(shang)(shang)）、離(li)蛹(yong)(yong)（附(fu)(fu)肢不緊貼在(zai)(zai)(zai)(zai)體(ti)上(shang)(shang)）和圍(wei)蛹(yong)(yong)（本(ben)質(zhi)上(shang)(shang)是(shi)(shi)離(li)蛹(yong)(yong)，但被(bei)幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)皮(pi)(pi)(pi)所形(xing)(xing)成的(de)(de)囊(nang)包圍(wei)）。蛻(tui)(tui)皮(pi)(pi)(pi)和變(bian)(bian)(bian)態都受(shou)激(ji)素(su)(su)(su)(su)(su)(su)的(de)(de)控制。蛻(tui)(tui)皮(pi)(pi)(pi)是(shi)(shi)由腦中(zhong)的(de)(de)神經(jing)分(fen)泌(mi)細胞(bao)(bao)產生(sheng)的(de)(de)激(ji)素(su)(su)(su)(su)(su)(su)發動的(de)(de)。這種激(ji)素(su)(su)(su)(su)(su)(su)作用于(yu)前胸(xiong)的(de)(de)一個(ge)內分(fen)泌(mi)腺(xian)(xian)－－前胸(xiong)腺(xian)(xian)，前胸(xiong)腺(xian)(xian)產生(sheng)蛻(tui)(tui)皮(pi)(pi)(pi)激(ji)素(su)(su)(su)(su)(su)(su)，作用于(yu)真(zhen)皮(pi)(pi)(pi)層，刺激(ji)生(sheng)長(chang)和形(xing)(xing)成表皮(pi)(pi)(pi)。變(bian)(bian)(bian)態同(tong)樣(yang)(yang)受(shou)激(ji)素(su)(su)(su)(su)(su)(su)控制：在(zai)(zai)(zai)(zai)整個(ge)幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)階段，腦后(hou)有(you)(you)一個(ge)小(xiao)腺(xian)(xian)體(ti)叫咽側體(ti)，分(fen)泌(mi)保(bao)(bao)幼(you)(you)激(ji)素(su)(su)(su)(su)(su)(su)。只要血(xue)液中(zhong)有(you)(you)保(bao)(bao)幼(you)(you)激(ji)素(su)(su)(su)(su)(su)(su)，正在(zai)(zai)(zai)(zai)蛻(tui)(tui)皮(pi)(pi)(pi)的(de)(de)真(zhen)皮(pi)(pi)(pi)細胞(bao)(bao)產生(sheng)幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)表皮(pi)(pi)(pi)。至最(zui)后(hou)一齡(ling)幼(you)(you)蟲(chong)(chong)(chong)(chong)(chong)(chong)時(shi)不再(zai)(zai)產生(sheng)保(bao)(bao)幼(you)(you)激(ji)素(su)(su)(su)(su)(su)(su)，于(yu)是(shi)(shi)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)(chong)變(bian)(bian)(bian)態成為(wei)(wei)(wei)(wei)成蟲(chong)(chong)(chong)(chong)(chong)(chong)。在(zai)(zai)(zai)(zai)全(quan)變(bian)(bian)(bian)態昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)(chong)中(zhong)，蛹(yong)(yong)在(zai)(zai)(zai)(zai)有(you)(you)極(ji)少量保(bao)(bao)幼(you)(you)激(ji)素(su)(su)(su)(su)(su)(su)的(de)(de)情(qing)況下發育(yu)。滯育(yu)雖然在(zai)(zai)(zai)(zai)任何蟲(chong)(chong)(chong)(chong)(chong)(chong)期都能(neng)發生(sheng)，但在(zai)(zai)(zai)(zai)蛹(yong)(yong)期最(zui)為(wei)(wei)(wei)(wei)常見(jian)。在(zai)(zai)(zai)(zai)溫帶，許多(duo)昆(kun)(kun)蟲(chong)(chong)(chong)(chong)(chong)(chong)以(yi)蛹(yong)(yong)期越(yue)冬。
除了(le)發(fa)育時形態(tai)的(de)(de)(de)(de)變(bian)化外，許(xu)(xu)多昆蟲(chong)(chong)(chong)(chong)(chong)成(cheng)蟲(chong)(chong)(chong)(chong)(chong)有多態(tai)現象。例如：工(gong)(gong)蟻(yi)和后蟻(yi)，工(gong)(gong)蜂和后蜂均(jun)不同；白(bai)蟻(yi)有兵蟻(yi)、繁殖(zhi)蟻(yi)和持續的(de)(de)(de)(de)幼蟲(chong)(chong)(chong)(chong)(chong)；蚜蟲(chong)(chong)(chong)(chong)(chong)成(cheng)蟲(chong)(chong)(chong)(chong)(chong)則有無翅(chi)和有翅(chi)之分(fen)；有些(xie)蝴(hu)蝶有引人注目(mu)的(de)(de)(de)(de)季節兩態(tai)性。這(zhe)些(xie)差別可(ke)解釋為：每一種(zhong)的(de)(de)(de)(de)每個(ge)成(cheng)員的(de)(de)(de)(de)基因(yin)中都有發(fa)育成(cheng)不同型的(de)(de)(de)(de)能力(li)，由于環境刺激引起特(te)定(ding)的(de)(de)(de)(de)發(fa)育途徑。激素(su)或許(xu)(xu)是控制這(zhe)些(xie)變(bian)化的(de)(de)(de)(de)環節。多數(shu)昆蟲(chong)(chong)(chong)(chong)(chong)營有性生(sheng)(sheng)殖(zhi)。蝴(hu)蝶的(de)(de)(de)(de)視力(li)很重要(yao)，雌(ci)(ci)蝶的(de)(de)(de)(de)色澤在飛行(xing)中能吸引同種(zhong)的(de)(de)(de)(de)雄(xiong)(xiong)蝶。蜉蝣(you)和有些(xie)蠓的(de)(de)(de)(de)雄(xiong)(xiong)蟲(chong)(chong)(chong)(chong)(chong)成(cheng)群飛舞吸引雌(ci)(ci)蟲(chong)(chong)(chong)(chong)(chong)。某些(xie)雌(ci)(ci)甲蟲(chong)(chong)(chong)(chong)(chong)的(de)(de)(de)(de)部(bu)分(fen)脂肪(fang)體形成(cheng)一個(ge)發(fa)光的(de)(de)(de)(de)器官吸引雄(xiong)(xiong)蟲(chong)(chong)(chong)(chong)(chong)。雄(xiong)(xiong)蟋蟀和蚱(zha)蜢發(fa)聲吸引雌(ci)(ci)蟲(chong)(chong)(chong)(chong)(chong)，雄(xiong)(xiong)蚊則被雌(ci)(ci)蚊飛行(xing)時發(fa)出的(de)(de)(de)(de)聲音所吸引。但(dan)最(zui)重要(yao)的(de)(de)(de)(de)是氣味(wei)。大多數(shu)雌(ci)(ci)蟲(chong)(chong)(chong)(chong)(chong)分(fen)泌信息素(su)引誘雄(xiong)(xiong)蟲(chong)(chong)(chong)(chong)(chong)，雄(xiong)(xiong)蟲(chong)(chong)(chong)(chong)(chong)同樣地也能產生(sheng)(sheng)吸引雌(ci)(ci)蟲(chong)(chong)(chong)(chong)(chong)的(de)(de)(de)(de)氣味(wei)。
交配和(he)(he)產卵(luan)(luan)(luan)需(xu)要(yao)適當的(de)溫(wen)度(du)和(he)(he)營養(yang)。一次(ci)受(shou)孕(yun)，終身(shen)產卵(luan)(luan)(luan)。交配、產卵(luan)(luan)(luan)期間對蛋(dan)(dan)白(bai)(bai)質(zhi)尤(you)其需(xu)要(yao)，鱗翅(chi)目的(de)成蟲(chong)(chong)只吃(chi)糖和(he)(he)水(shui)，幼(you)蟲(chong)(chong)時貯備下必需(xu)的(de)蛋(dan)(dan)白(bai)(bai)質(zhi)。溫(wen)度(du)和(he)(he)營養(yang)常(chang)影(ying)響激(ji)素的(de)分(fen)泌(mi)(mi)。產卵(luan)(luan)(luan)時通常(chang)需(xu)要(yao)保幼(you)激(ji)素或來自神經分(fen)泌(mi)(mi)細(xi)胞(bao)的(de)激(ji)素。沒有這些(xie)激(ji)素，則生殖中(zhong)斷。這些(xie)現象在(zai)(zai)冬季見于馬鈴(ling)薯(shu)甲蟲(chong)(chong)。少數(shu)昆蟲(chong)(chong)雄蟲(chong)(chong)罕見，由雌蟲(chong)(chong)進(jin)行孤(gu)雌生殖。溫(wen)帶的(de)蚜(ya)蟲(chong)(chong)在(zai)(zai)夏(xia)季只產生營孤(gu)雌生殖的(de)雌蚜(ya)，胚(pei)胎(tai)(tai)在(zai)(zai)母蚜(ya)內發(fa)(fa)育（胎(tai)(tai)生）。某些(xie)癭蚊幼(you)蟲(chong)(chong)的(de)卵(luan)(luan)(luan)巢(chao)中(zhong)卵(luan)(luan)(luan)母細(xi)胞(bao)能在(zai)(zai)孤(gu)雌情(qing)況(kuang)下開始發(fa)(fa)育，幼(you)體破壞母蟲(chong)(chong)體壁逸出，這叫幼(you)體生殖。
