Like other beetles, fireflies' forewings are hardened as elytra, for protecting the hindwings and the abdomen. Male fireflies need highest mobility to search for female, both wings are fully developed and functional. Female of some firefly species only have reduced wings. In some species, wings are completely lacking.

Abdomen segmented, each segment contains tergite and ventrite, number, shape, size and structure of which are important for classification and identification of different firefly species. In Rhagophthalmidae, male has 8 visible abdominal ventrites and female has 9. In Luciolinae, there are 6 visible ventrites in male and 7 visible ventrites in female. In Lampyrinae, Ototretinae and Cyphonocerinae, male has 7 or 8 visible ventrites. Macropterous (wings fully developed) females have 6 or 7 ventrites. Females with reduced or lost wings have 8 visible abdominal segments.

發光器的大小、形狀、位置、和數目，在不同品種之間和同種的雌雄之間可以有很大的差異。發光器可以是一圓點、一片、C形、條狀、橢圓形、長方形或者佔據整片腹板。雌性雙色垂鬚螢近乎全身發光。數目方面，可以是1對、2對、3對、以致33點 (每節3點, 雌性凹眼螢)

The light emitting part is called lantern, luminescent organ, photic organ, photogenic organ or light organ. Size, shape, location and number of light organ could vary greatly between different species or between genders of the same species. The light organ could be a spot, a patch, c-shaped, a strip, oblong, rectangular or occupying the whole ventrite. In female Stenocladius bicoloripes, almost the whole body glows. There could be only one pair, two pairs, three pairs, or even 33 light spots (3 for each segment) as in female Rhagophthalmus

幼螢通常以黑、褐、黃等與生活環境比較接近的顏色為主，部份品種也會有紅黑配搭的警戒色。
幼蟲多是長形、狹長、前後兩端收窄。身體由頭、三節胸部和九節腹部組成。背面骨化比較明顯、比較硬。體節間膜和腹面的彈性容許身體伸長、縮短卷曲。有些幼蟲可以"蛭式"步行。

頭部可縮入前胸背板下，探測環境和獵食時向前伸出。觸角短、三節；單眼位於兩則。幼蟲不具複眼，而單眼只能感應光暗變化，不能成像。口器非常發達，用於獵食。

陸生幼蟲第1-8腹節的每對側背片有各有1氣孔，在水生幼蟲身上，氣孔被鰓取代。

相比其他品種，窗螢幼蟲具有特別發達的足，這與些屬幼蟲的快速行動和攀爬樹枝能力有關。窗螢幼蟲更會有足抓緊獵物蝸牛的殼。

很多螢火蟲幼蟲具翻縮腺，用以分泌化學物驅散侵襲者。受刺激時，翻縮腺從體節兩側伸出，通常很快縮回體內。

香港已知的全部螢火蟲幼蟲都具備發光器，位於第7腹節或第8腹節，有時兩節皆有發光器。幼蟲的發光，對潛在捕食者乃警戒訊號

腹部未端有一束可翻縮的組織，稱為尾足，尾足用作固定身體、協助步行、捕捉獵物和清潔身體。

Colour of the larvae is usually similar to the living environment, such as black, brown and yellow. A few species have combination of red and dark colour which could be a warning signal to their potential predator. Larvae are generally elongated, slender and more or less tapering in both ends. The body is composed of head, three thoracic and nine abdominal segments. Dorsal side is usually more sclerotized and harder than the ventral side of the body. Intersegmental membranes and flexibility of the ventral part allow certain degree of elongation, shortening and curling of the body. Some larvae can even perform leech-like looping movement

Head is retractable beneath the pronotum and extensible when searching for preys or attacking preys. Antennae short, 3-segmented; ocelli (simple eyes) located on each side. Firefly larvae do not have compound eyes. Ocelli could only sense the brightness level and have no imaging capability. Mouthparts are well developed for predation.

Terrestrial larvae bear a pair of spiracles on the latero-tergites of abdominal segment 1-8. The spiracles are replaced by gills in true aquatic larvae.

Pyrocoelia has much longer and better developed legs compared with other species. This may be associated to their highly mobile behavior and ability of climbing on twigs. Their legs are also frequently used to grasp the snail shell when insect their head into the shell .

Many larvae also bear eversible glands which secret repellent substances. These eversible glands are only occasionally extruded for a short period of time from the lateral sides of each segment when the larva is under stimuli.

Light organs are found in all the recorded larvae from Hong Kong. Light organs exist at abdominal segment 7 , abdominal segment 8 or in some cases at both segment 7 and 8. Light signal emitted by the larvae is an aposematic display to their potential predators

There are a series of eversible filaments at the terminal abdominal segment called pygopods. Pygopods are used as a holdfast for anchorage and assisting locomotion They are also useful for holding the shell of preys, as well as cleaning the body.

窗螢幼蟲蛭式行走 Leech-like Looping Movement of Window a Window Firefly Larva from Hong Kong Fireflies on Vimeo.