How Do I Find Short Interest for a Stock?

By Larsen Kusick
Where do you find the percentage of shorts in the float of a stock? - D.
I'm a big believer in the importance of investors knowing all about the short position in a stock before making a purchase. I'm going to answer D's question first, and then explain what the percentage of shorts in the float of a stock is and why it's important. I'm also going to assume that readers know what it means to take a short position in a stock, and understand what it means when I say that a certain number of shares of a stock are being "shorted." (If you need a quick refresher on shorting, or "short-selling," check out Stocks: Shorting.)

Where You Can Find Short Interest Data

You can find data regarding the short position in a stock in a number of places. A good place to start is Nasdaq.com, which provides short interest data for stocks listed on the Nasdaq, as well as the New York Stock Exchange (NYSE) and the American Stock Exchange (Amex). The site is particularly helpful because it provides short interest data for each of the past 12 months, so you can see if the short position in a stock has been increasing or decreasing over the past year. However, Nasdaq.com only provides the number of shares shorted each month, as opposed to the "percentage of shorts in the float of a stock" that D. is looking for.

Where You Can Find the 'Percentage of Shorts in the Float of a Stock'

The easiest place to find this information is by putting a ticker into Yahoo! Finance and clicking on the "Key Statistics" link. Scroll down a bit and look at the right column under "Share Statistics" -- here you'll find the most recent data for:
* Number of shares shorted
* Short ratio
* Short position as a percentage of the float
* Number of shares shorted during the previous month

The number of shares shorted should be the same as the information from Nasdaq.com.

The short ratio, also referred to as "days to cover," is an expression of how large the total short position is in a stock relative to the average daily volume.

The short position as a percentage of the float is the information that D. is looking for. This percentage expresses the number of shares shorted divided by the "float" (Float Defined float), or number of total shares that are free to be traded by the investing public.
f the percentage of shorts in a stock were to hit 100% that would mean that every single available share of a stock had been sold and would have to be bought back before being available to be sold again. (Note: In reality, 100% would be effectively impossible to see, but it is useful to explain this concept.)
Motivated Buyers

Anyone who takes a short position in a stock is entering an interesting situation: In order to exit the position, he or she has to "cover" or buy back the shares that are being shorted. So if a stock has a very high percentage of its shares being shorted, it means that there are more investors who need to buy shares at some point, whether the stock goes up or down.

This is interesting because those who have already taken a short position need those who own the stock (or are "long") to sell their shares and push the price lower.

When Good News Is Actually Bad News

If a company comes out with positive news and more investors want to buy a stock -- pushing the price higher -- those with short positions can rack up losses quickly. For this reason, stocks with high short ratios are more prone to a big upward move, called a "short squeeze," in which what would have been a smaller rise in the share price is exacerbated by short covering, as those who are "short a stock" panic and buy back shares to cut their losses. (In November 2006, I wrote about a short squeeze in Crocs (CROX_) -- A Tight Fit for Crocs' Shorts.)

Short Percentages in 'Safer' Stocks

Large-cap blue-chip stocks tend to have an extremely low short position relative to their float. This is due to a number of factors, including the sheer amount of stock that is being held my mutual funds and other institutions, as well as the simple fact that anyone looking for a stock that could go down significantly will generally not be interested in these stocks, which are safer and significantly correlated to the broad economy.
Think of it this way: If something were to happen to push General Electric down 10% or more, there are much more risky stocks out there that would go down a lot more on that day. Here are some well-known blue chips and the recent short position as a percentage of the float:
'Safer' Stocks and Their Short Positions
General Electric (GE) 0.2%
Coca-Cola (KO) 0.4%
Anheuser Busch (BUD) 0.6%
Wal-Mart (WMT) 0.7%
Microsoft (MSFT) 0.9%
McDonald's (MCD) 1.2%
Google (GOOG) 1.6%
Short Percentages in 'Riskier' Stocks

You'll find higher short percentages in riskier stocks. There are a number of reasons for an investor or trader to take a big short position in a stock. Some may think a stock has gone up too much and is set for a fall, while others may see a struggling company with a falling stock and are willing to bet that it will go down further. Some simply believe that the valuation for a stock is too high, while some are predicting an earnings shortfall in the near future.

There's no "typical" percentage for investors to compare a stock to, but the list below shows a number of companies, some good and some bad, and the recent short position as a percentage of the float:

'Riskier' Stocks and Their Short Positions
Amgen (AMGN) 2.4%
Marvell Technology (MRVL) 3.9%
Akamai (AKAM) 5.9%
Whole Foods (WFMI) 7.5%
XM Satellite Radio (XMSR) 9.7%
Ford (F) 10.2%
Amazon.com (AMZN) 14.9%
Palm (PALM) 18.6%
Under Armour (UA) 28.0%
NutriSystem (NTRI) 33.2%
Crocs (CROX) 35.2%

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朱德庸放牛吃草 獨子建中台大

【聯合報╱本報記者陳宛茜】

教養更重要朱德庸（右）、馮曼倫夫婦寶貝兒子，卻是從小就訓練孩子獨立。 圖／朱德庸提供

教養更重要這是兒子小時候自己規畫的評量表。 記者高智洋／攝影

朱家教養方式獨樹一格，朱重威在同學眼中，也一直是個與眾不同的學生。朱德庸說，兒子的成長過程中，經常受到同儕的壓力，遭遇認同危機。這時夫妻倆會為孩子分析、給他信心。

某年朱重威班上同學都在瘋電子寵物，老師不准他們把寵物機帶到課堂上。朱重威和同學一樣，把電子寵物交給老爸代養。沒想到他放學回家，卻發現老爸什麼都沒做，電子寵物也死了。

這是朱德庸教養孩子的方式，讓他明白「沒時間養，就不要養」，從中學會責任感。電子寵物死後，朱德庸還為孩子上堂「生命教育」，告訴孩子電子世界的虛幻，帶著他與大自然接觸，最後兒子不再迷戀電子寵物。

朱德庸認為，青少年養電子寵物、沉迷網咖，多半是因為「朋友都在玩」，未必是真心喜愛。「台灣父母如果肯在教養下工夫，小孩就不會盲目追求流行、陷入集體的狂熱。」朱德庸擔憂，這一代父母沒有告訴孩子「生命的基本價值」，讓他們明白虛擬世界的虛幻。

夫妻從小灌輸朱重威「不必為父母上課」、「長大做工人也沒關係」的觀念，被放牛吃草的朱重威卻一路考上建中、台大。

馮曼倫說，關鍵點在國中。朱重威原本讀公立國中，但她發現學校的政策是把每個人都當最後一名的學生來教，兒子上課輕鬆、卻學不到東西。夫妻花了很長時間告 訴他，若想成為自己想成為的人，必須培養什麼樣的競爭力。朱重威想了很久，決定轉學到壓力大的私立國中。後來他考上建中、甄試上台大昆蟲系，統統都是他自 己設定目標。

朱德庸說，兒子國、高中成績都不算理想，卻總能在最後關頭衝刺成功、念到心中的「第一志願」。「他總是比別人花更多時間確定目標，花更多時間尋找自己達到 目標的方式；但只要找到了，他就可以比別人更快到達。」朱德庸說，這是因為夫妻倆從小就教導兒子「做自己、為自己負責！」

律師的獨子──相當感人的真人故事

我的爸爸是任何人都會引以為榮的人。

他是位名律師，精通國際法，客戶全是大公司，因此收入相當好。

可是他卻常常替弱勢團體服務，替他們提供免費的服務。不僅如此也，他每週都有一天會去勵德補習班去替那些青少年受刑人補習功課，每次高中放榜的時候，他都會很緊張地注意有些受刑人榜上是否有名。

我是獨子，當然是三仟寵愛在一身，爸爸沒有慣壞我，可是他給我的實在太多了。

我們家很寬敞，也佈置得極為優雅。爸爸的書房是清一色的深色傢俱、深色的書架、深色的橡木牆壁、大型的深色書桌、書桌上造型古雅的燈，爸爸每天晚上都要在他書桌上處理一些公事，我小時常乘機進去玩。

爸爸有時也會解釋給我聽他處理某些案件的邏輯。

他的思路永遠是如此合乎邏輯，以至我從小就學會了他的那一套思維方式，也難怪每次我發言時常常會思路很清晰，老師們當然一直都喜歡我。

爸爸的書房裡放滿了書，一半是法律的，另一半是文學的，爸爸鼓勵我看那些經典名著。

因為他常出國，我很小就去外國看過世界著名的博物館。

我隱隱約約地感覺到爸爸要使我成為一位非常有教養的人，在爸爸的這種刻意安排之下，再笨的孩子也會有教養的。

我在唸小學的時候，有一天在操場上摔得頭破血流。老師打電話告訴了我爸爸。

爸爸來了，他的黑色大轎車直接開進了操場，爸爸和他的司機走下來抱我，我這才注意到司機也同樣穿著黑色的西裝，我得意得不得了，有這麼一位爸爸，真是幸福的事。

我現在是大學生了，當然一個月才會和爸媽渡一個週未。前幾天放春假，爸爸叫我去墾丁，在那裡我家有一個別墅。爸爸邀我去沿著海邊散步，太陽快下山了，爸爸在一個懸崖旁邊坐下休息。

他忽然提到最近被槍決的劉煥榮，爸爸說他非常反對死刑，死刑犯雖然從前曾做過壞事，可是他後來已是手無寸鐵之人，而且有些死刑犯後來完全改過遷善，被槍決的人，往往是個好人

我提起社會公義的問題，爸爸沒有和我辯論，只說社會該講公義，更該講寬恕。他說：我們都有希望別人寬恕我們的可能'。

我想起爸爸也曾做過法官，就順口問他有沒有判個任何人死刑 ？

爸爸說：我判過一次死刑，犯人是一位年青的原住民，沒有什麼常識，他在台北打工的時候，身份証被老闆娘扣住了，其實這是不合法的，任何人不得扣留其他人的身份証。

他簡直變成了老闆娘的奴工，在盛怒之下，打死了老闆娘。我是主審法官，將他判了死刑。

事 後，這位犯人在監獄裡信奉了宗教，從各種跡象來看，他已是個好人，因此我四處去替他求情，希望他能得到特赦，免於死刑，可是沒有成功。他被判刑以後，太太 替他生了個活潑可愛的兒子，我在監獄探訪他的時候，看到了這個初生嬰兒的照片，想到他將成為孤兒，也使我傷感不已，由於他已成另一個好人，我對我判的死刑 痛悔不已。

他臨刑之前，我收到一封信。

爸爸從口袋中，拿出一張已經變黃的信紙，一言不發地遞給了我。

信是這樣寫的：

法官大人：

謝謝你替我做的種種努力，看來我快走了，可是我會永遠感謝你的。

我有一個不情之請，請你照顧我的兒子，使他脫離無知和貧窮的環境，讓他從小就接受良好的教育，求求你幫助他成為一個有教養的人，再也不能讓他像我這樣，糊裡糊塗地浪費了一生。

ＸＸＸ敬上

我對這個孩子大為好奇，爸爸你怎麼樣照顧他的孤兒，爸爸說：我收養了他。

一瞬間，世界全變了。

這不是我的爸爸，他是殺我爸爸的兇手，子報父仇，殺人者死。

我跳了起來，只要我輕輕一推，爸爸就會粉身碎骨地跌到懸崖下面去。

可是我的親生父親已經寬恕了判他死刑的人，坐在這裡的，是個好人，他對他自已判人死刑的事情，始終耿耿於懷，我的親生父親悔改以後，仍被處決，是社會的錯，我沒有權利再犯這種錯誤。

如果我的親生父親在場，他會希望我怎麼辦 ？

我蹲了下來，輕輕地對爸爸說：「爸爸，天快黑 了，我們回去吧！媽媽在等我們。」

爸爸站了起來，我看到他眼旁的淚水，兒子，謝謝你，沒有想到你這麼快就原諒了我。

我發現我自己的眼光也因淚水而有點模糊，可是我的話卻非常清晰 ：「爸爸，我是你的兒子，謝謝你將我養大成人。」

海邊這時正好刮起了墾丁常有的落山風，爸爸忽然顯得有些虛弱，我扶著他，在落日的餘暉下，向遠處的燈光頂著大風走回去，荒野裡只有我們父子二人。

我以我死去的生父為榮，他心胸寬大到可以寬恕判他死刑的人。
我以我的爸爸為榮，他對判人死刑，一直感到良心不安，他已盡了他的責任，將我養大成人，甚至對我可能結束他的生命，都有了準備。

而我呢？我自已覺得我又高大、又強壯，我已長大了。

只有成熟的人，才會寬恕別人，才能享受到寬恕以後而來的平安，小孩子是不會懂這些的。

我的親生父親，你可以安息了。

你的兒子已經長大成人， 我今天所做的事，一定是你所喜歡的 。

---------感言--------

各位朋友：

還記得小時候一件轟動社會的山地青年殺死雇主的案子嗎？

如 果我沒記錯，那位山地青年應該叫做湯英申，純樸的山地青年來到繁華的台北打工，碰了惡劣的雇主，不僅壓榨勞力還剝奪自由，在受盡各種不平等的待遇後，選擇 了採取最激烈的行動以示抗議，雖然手段並不正確，但卻情有可原，死刑或能嚇阻犯罪，但對於一個走投無路且深具悔意的人來說或許太沉重了，雖然社會各界的呼 籲請求，希望獲得特赦，但仍免不了這樣的結局，故事的結局雖然令人遺憾；但事隔近二十年後，卻有如此令人驚喜的發展，這不為人知的一幕，包含了多少的寬容 與救贖，我想這就是所謂的大愛吧！

灼热的生命

灼热的生命
作词：王新莲 作曲：林琼珑/韩贤光 编曲：Chris Babida

燃烧 灼热的生命 慢慢地烧

让我睁大眼睛 把一切都看清

把烟雾全部拨开 无悔无恨地走下去

燃烧 灼热的生命 慢慢地烧

过去现在未来 全在我心里

什么轮回我不管 浴火重生新的自己

喔～ 山高水深 云淡风清

生命的章法 复杂的一如繁星

火凤凰的愿望 是刹那间的那个决定

而我奔窜的血液 正是那件火红的外衣

燃烧 灼热的生命 慢慢地 慢慢地烧

燃烧 灼热的生命 慢慢地 慢慢地烧

燃烧 灼热的生命 慢慢地烧

过去现在未来 全在我心里

什么轮回我不管 浴火重生新的自己

喔～ 山高水深 云淡风清

生命的章法 复杂的一如繁星

火凤凰的愿望 是刹那间的那个决定

而我奔窜的血液 正是那件火红的外衣

燃烧 灼热的生命 慢慢地 慢慢地烧

燃烧 灼热的生命 慢慢地 慢慢地烧

燃烧 灼热的生命 慢慢地 慢慢地烧

燃烧 灼热的生命 慢慢地 慢慢地烧

國共合體 他一身歷史印記

【聯合報╱記者李蕙君／台東縣報導】 2009.10.02 03:19 am

王忠明手部、胸部、背部還留有當初自願撤退到台灣，在身上刺反共等字的標語。 記者李蕙君／攝影 台東市八十一歲榮民王忠明，娶妻的隔天就離家出征勦匪去了，結果受傷被俘成了共軍，參加韓戰抗美援朝失敗被送到台灣，再次成為國軍。他怕拖累大陸家人而改名，如今體內還留著勦匪受傷時的彈殼。 昨天，是中國建政六十周年，看著電視直播中共閱兵畫面，對照自己胸前、手臂多處「殺朱拔毛」的反共刺青，走過一甲子的他，有時空錯置的感覺，嘆自己的遭遇是「歷史的錯誤」。 「少小離家卻不能老大回，」王忠明說，即使目前兩岸來往頻繁，回山東老家不難，「我還記得『王宗祿』娶老婆的隔天就離家出征了，家裡的地址都還記得；但人事已非，寄了好幾十封信都沒音信，我是回不去了！」 原名王宗祿的王忠明，十六歲離家征戰，被共軍俘虜，後來派往朝鮮支援韓戰。戰敗後，他選擇追求自由來到台灣，被視為反共義士，同行還有一萬四千多人。 他說為表明反共決心與忠誠，來台前，拿著織麻布袋的大頭針，在身上刺「三民主義、建國救世」「殺朱拔毛、報仇雪恨」等刺青；憂心家人遭追殺，不得不改名，又怕忘了本名，因此在左手刺上本名「王宗祿」。 回憶過往征戰，最慘烈莫過勦匪時中彈，瞬間血流如注。他說：「子彈從左胸穿過左背爆開一個大彈孔，彈頭還在體內，醫生說開刀有危險，一直沒拿，現在身體偶爾會痠痛。」 來台後，他還參加八二三砲戰，常說：「我的生日有很多天，只要是戰後存活的日子都是生日。」五十八年他退伍，擔任公路局司機，在台東展開新生活。 回首過往，他說：「那是一場不得不打的戰爭（國共內戰），多少兄弟當時一個是共軍，一個是國軍，兩相廝殺，沒什麼對錯，就是自保而已。」

（時間子句式的）分詞構句可予以「減化」嗎？

http://www.yentzu.idv.tw/yen_talk.asp?my_id=55

也請參考 副詞子句連接詞
http://tw.myblog.yahoo.com/jw!uvuF2ZieGRZJuB4yD_bOH6qQO9E-/article?mid=325&prev=330&next=324

由於有不少讀者問到「分詞構句」的「減化」問題，我現在把我書中關於這一部份的說明略為簡化並稍加一點說明放在這裡供大家參考：

A. 「背景子句」與「時間子句」的不同

最近幾年市面上有位名氣頗大並且兼某英語雜誌總主筆的補教名師出版了一本非常暢銷的英文文法書「...魔法師之文法俱樂部」，其中便用所謂「減化」的觀念來解釋分詞構句。

下面便是這位先生的「減化」理論中最重要的一環—–「副詞子句的減化」—–的一個典型三部驟。請讀者思考一下，這個理論有沒有什麼不妥的地方？

思考：1. While he was lying on the couch, the boy fell asleep. （這男孩躺在沙發上時，睡著了。）

可以減化成：

2. While lying on the couch, the boy fell asleep. （這男孩躺在沙發上時，睡著了。）

更可再減化成：

3. Lying on the couch, the boy fell asleep.

英語文法學者（譬如Azar Shrampfar所寫的Understanding and Using English Grammar）都把第二句視為是第一句的「子句簡化」（reduced clauses），這點沒錯。至於第三句則由於「通常」是與第一、二句意義不同的另一種句子，因此並不被文法學者視為是前兩句的「簡化」。

然而國內這位寫「魔法書」的先生則說，「第二句在省略掉While之後，句意仍很清楚，所以2句「減化」成3句是很自然的。」他不了解為什麼「傳統」的文法學者會把 Lying on the couch...這樣的句型稱為「分詞構句」，而認為他自己發明了一個前人所未能見及的文法解釋理論。他說：「了解減化子句的來龍去脈之後，就會了解「分詞構句」一詞實在是多此一舉。lying on the couch本來就是副詞子句While he was lying on the couch的減化，無需用任何特別名稱來表示。」

這位先生之所以會有這樣的誤解，乃是因為他不了解分詞構句的功能–—–這是缺乏寫作經驗....的結果。

所謂「縮減子句」（reduced clauses）乃是寫作者為了修辭的「精簡」而將一些子句的句型「精簡」成片語句型。我們在前面已經向讀者介紹過。此地我們且先來談上面的第三句 Lying on the couch, the boy fell asleep.為什麼不是前面兩句的簡化。

傳統文法學者之所以不將上述第三句視為是一、二兩句的簡化乃是因為一、二兩句跟三句的意思並不相同。一、二兩句只是一個單純的時間副詞子句，說明「這個男孩」睡著的「時間」，而三句所企圖要說明的則有可能是「這個男孩」睡著的「原因」。

以分詞構句開頭做引導的句型，如上面的三句，一般乃是用來做為原因或背景說明的。（※參見前節「代替原因子句」一段說明）

Lying on the couch, the boy fell asleep.

躺在沙發上，這孩子睡著了。

這句話所意含的有可能是，這孩子原先並不是躺在沙發上，或許原先是在地上，或是被大人背著，之後放到沙發上才睡著。它是為了與「其他情況」做比較所做的說明，跟一、二兩句之只是單純說明時間是完全不同的。上面的句子由於太短，所以這種「背景說明」的性質並不明顯，讀者若是將之擴充成下句的情況，就可以很清楚的明白這個道理。

Lying on the (soft cushion of the) couch, the boy (soon) fell asleep.

躺在沙發（軟綿綿的墊子）上，這孩子（很快）的就睡著了。

我們只有在極少數不影響句意的情況下，可以將時間子句連接詞while省略，譬如下句：

While walking/Walking on Dun-Hwa South Road yesterday, I came across an old friend whom I haven’t seen for years.

昨天我走在敦化南路上時，碰到一位多年不見的老朋友。

這是因為「走在路上」這種事情甚為單純，在這一句中不太可能會有什麼作用使之可以成為主要子句的背景，所以不太影響句意，從而可以「省略」（※新加說明：按，這裡的While之所以能省略，主要也是因為主要子句中已有另一個時間副詞yesterday所以能省略）。下句則不同：

Walking along Taipei’s tree-lined Dun-Hwa South Road, one can see quite a few really nice European style coffee shops.

走在台北敦化南路的林蔭道上，你可以看到不少真的很不錯的歐式咖啡館。

這個句子的分詞構句所要說的便是「原因」（只有走在敦化南路你才能看見），而非是「時間」，因此跟While walking...是不同的。

這種「時間」（temporal）子句與「因由」（causal）子句的不同不知為何會有許多國人不能分別。最近我在一份七八年前出版的GMAT暢銷舊講義中也看到作者所舉的一個說明「絕對分詞構句」的例句：

（When he was five years old, his father died. =）

He being five years old, his father died. (??)

（由於）他五歲，他的父親死了。（??）

「分詞構句」都是用來說明「主要子句」所敘述事件「之所以會/可能如此...」的緣由的，即使是「絕對（「自由」）分詞構句」亦然。「絕對分詞構句」雖然在文法上不必跟主要子句有所關聯，在文意上卻仍然不能跟主要子句離題。像上述這樣純粹「時間性」的句子並不宜改用「分詞構句」——不管是否「絕對分詞」構句 ——來寫，因為「他五歲」跟「他的父親死了」兩者之間除了時間上的「重疊」以外，並不可能互相有所「影響」。我看這位作者是位學養俱豐的長者，應該不致犯此錯誤，或許是「信手拈來」一個句子，並且未仔細校稿，致有此誤吧。總之，類似這樣的句子通常所要說的應該比較可能是類似如下的「故事」：

His father having died and he being only five years old, his mother had to leave him in the care of a relative and go up north to look for jobs.

由於他的父親過世，而且他才五歲，他媽媽不得不將他托給一個親戚照顧，之後北上找工作。

Blink Twice if You Like me

MATING In many fireflies, pairs stay coupled for hours while the male, lower, gives the female a protein package injected with sperm, called a nuptial gift.

Melody Ko/Tufts University

RESEARCHER Sara Lewis is an evolutionary ecologist at Tufts University who studies firefly mating habits.

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Tufts University

SPIRALS A coiled nuptial gift may take up a lot of space in a male’s abdomen. Receiving a gift can make a difference in the female’s reproductive success.

VIDEO

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Firefly Courtship Signals

A video from Tufts University's Department of Biology on the energy and predation costs of firefly courtship signals.

LINCOLN, Mass. — Sara Lewis is fluent in firefly. On this night she walks through a farm field in eastern Massachusetts, watching the first fireflies of the evening rise into the air and begin to blink on and off. Dr. Lewis, an evolutionary ecologist at Tufts University, points out six species in this meadow, each with its own pattern of flashes.

Along one edge of the meadow are Photinus greeni, with double pulses separated by three seconds of darkness. Near a stream are Photinus ignitus, with a five-second delay between single pulses. And near a forest are Pyractomena angulata, which make Dr. Lewis’s favorite flash pattern. “It’s like a flickering orange rain,” she said.

The fireflies flashing in the air are all males. Down in the grass, Dr. Lewis points out, females are sitting and observing. They look for flash patterns of males of their own species, and sometimes they respond with a single flash of their own, always at a precise interval after the male’s. Dr. Lewis takes out a penlight and clicks it twice, in perfect Photinus greeni. A female Photinus greeni flashes back.

“Most people don’t realize there’s this call and response going on,” Dr. Lewis said. “But it’s very, very easy to talk to fireflies.”

For Dr. Lewis, this meadow is the stage for an invertebrate melodrama, full of passion and yearning, of courtship duets and competitions for affection, of cruel deception and gruesome death. For the past 16 years, Dr. Lewis has been coming to this field to decipher the evolutionary forces at play in this production, as fireflies have struggled to survive and spread their genes to the next generation.

It was on a night much like this one in 1980 when Dr. Lewis first came under the spell of fireflies. She was in graduate school at Duke University, studying coral reef fish. Waiting for a grant to come through for a trip to Belize, she did not have much else to do but sit in her backyard in North Carolina.

“Every evening there was this incredible display of fireflies,” Dr. Lewis said. She eventually started to explore the yard, inspecting the males and females. “What really struck me was that in this one-acre area there were hundreds of males and I could only find two or three females,” she said. “I thought, ‘Man, this is so intense.’ ”

When a lot of males are competing for the chance to mate with females, a species experiences a special kind of evolution. If males have certain traits that make them attractive to females, they will mate more than other males. And that preference may mean that those attractive males can pass down their traits to the next generation. Over thousands of generations, the entire species may be transformed.

Charles Darwin described this process, which he called sexual selection, in 1871, using male displays of antlers and feathers as examples. He did not mention fireflies. In fact, fireflies remained fairly mysterious for another century. It was not until the 1960s that James Lloyd, a University of Florida biologist, deciphered the call and response of several species of North American firefly.

Dr. Lewis, realizing that other firefly mysteries remained to be solved, switched to fireflies from fish in 1984, when she became a postdoctoral researcher at Harvard. She taught herself Dr. Lloyd’s firefly code and then began to investigate firefly mating habits. North American fireflies spend two years underground as larvae, then spend the final two weeks of their lives as adults, flashing, mating and laying eggs. When Dr. Lewis started studying fireflies, scientists could not say whether the females mated once and then laid all their eggs, or mated with many males. “Nobody knew what happened after the lights went out,” Dr. Lewis said.

She searched for mating fireflies in the evening, marked their locations with surveyor’s flags and then revisited them every half-hour through the night. They were still mating at dawn.

“It was cool to watch the sun rise and see the couples breaking up and the females crawling down the grass to lay their eggs,” Dr. Lewis said.

Many Americans are familiar with the kinds of fireflies Dr. Lewis studies, but they represent only a tiny fraction of the 2,000 species worldwide. And there is enormous variation in these insects. “There are some species that produce flashes when they’re adults, and there are some that simply glow as adults,” Dr. Lewis said. “Then there are a whole bunch of species where the adults don’t produce any light at all.”

In recent years scientists have analyzed the DNA of fireflies to figure out how their light has evolved. The common ancestor of today’s fireflies probably produced light only when they were larvae. All firefly larvae still glow today, as a warning to would-be predators. The larvae produce bitter chemicals that make them an unpleasant meal.

As adults, the earliest fireflies probably communicated with chemical signals, the way some firefly species do today. Only much later did some firefly species gain through evolution the ability to make light as adults. Instead of a warning, the light became a mating call. (An enzyme in the firefly’s tail drives a chemical reaction that makes light.)

The more Dr. Lewis watched firefly courtship, the clearer it became that the females were carefully choosing mates. They start dialogues with up to 10 males in a single evening and can keep several conversations going at once. But a female mates with only one male, typically the one she has responded to the most.

Dr. Lewis wondered if the female fireflies were picking their mates based on variations in the flashes of the males. To test that possibility, she took female fireflies to her lab, where she has computer-controlled light systems that can mimic firefly flashes. “You can play back specific signals to females and see what they respond to,” Dr. Lewis said.

The female fireflies turned out to be remarkably picky. In many cases, a male flash got no response at all. In some species, females preferred faster pulse rates. In others, the females preferred males that made long-lasting pulses.

If females preferred some flashes over others, Dr. Lewis wondered why those preferences had evolved in the first place. One possible explanation was that the signals gave female fireflies a valuable clue about the males. Somehow, mating with males with certain flash patterns allowed females to produce more offspring, which would inherit their preference.

It is possible that females use flashes to figure out which males can offer the best gifts. In many invertebrate species, the males provide females with food to help nourish their eggs. Dr. Lewis and her colleagues discovered that fireflies also made these so-called nuptial gifts — packages of protein they inject with their sperm.

Dr. Lewis is not sure why she and her colleagues were the first to find them. The gifts form coils that can take up a lot of space in a male firefly’s abdomen. “They’re incredibly beautiful,” she said.

Receiving nuptial gifts, Dr. Lewis and her colleagues have shown, can make a huge difference in the reproductive success of a female firefly. “It just about doubles the number of eggs a female can lay in her lifetime,” she said. One reason the effect is so big is that fireflies do not eat during their two-week adulthood. A slowly starving female can use a nuptial gift to build more eggs.

In at least some species, females may use flashes to pick out males with the biggest gifts. Dr. Lewis has tested this hypothesis in two species; in one, males with conspicuous flashes have bigger gifts. In another species, she found no link.

“In some cases they could be honest signals, and in some cases they could be deceptive signals,” Dr. Lewis said.

Deception may, in fact, evolve very easily among fireflies. It turns out that a male firefly does not need to burn many extra calories to make flashes. “It takes some energy, but it’s tiny. It’s less costly for a male than flying around,” Dr. Lewis said.

If making light is so cheap for males, it seems odd that they have not all evolved to be more attractive to females. “What is it that keeps their flashes from getting longer and longer or faster and faster?” Dr. Lewis asked.

Scanning the meadow, she grabbed her insect net and ran after a fast-flying firefly with a triple flash. She caught an animal that may offer the answer to her question. Dr. Lewis dropped the insect into a tube and switched on a headlamp to show her catch. Called Photuris, it is a firefly that eats other fireflies.

“They are really nasty predators,” Dr. Lewis said. Photuris fireflies sometimes stage aerial assaults, picking out other species by their flashes and swooping down to attack. In other cases, they sit on a blade of grass, responding to male fireflies with deceptive flashes. When the males approach, Photuris grabs them.

“They pounce, they bite, they suck blood — all the gory stuff,” Dr. Lewis said. She has found that each Photuris can eat several fireflies in a night. Photuris kills other fireflies only to retrieve bad-tasting chemicals from their bodies, which it uses to protect itself from predators.

To study how Photuris predation affects its firefly prey, Dr. Lewis and her colleagues built sticky traps equipped with lights that mimicked courtship signals of Photuris’s victims. The scientists found that Photuris was more likely to attack when flash rates were faster. In other words, conspicuous flashes — the ones females prefer — also make males more likely to be killed.

“At least where Photuris predators are around,” Dr. Lewis said, “there’s going to be a strong selection for less conspicuous flashes.”